In my campaign, I have one Navigator (PC), one Astropath and one Astropath assistant (NPCs). I think a single Astropath without a choir, or, in my case, assistant, would far too often lack the psychic strength required for communicating over the vast distances of the expanse.

Concerning Navigators, I would put no more than two on one ship: One primary navigator and maybe one apprentice. Many people state that loosing the only navigator means losing the ship, an ancient and unbelievably valuable piece of technology. That's true, but I'm not sure whether the ship is worth more than the Navigator or the other way round.

The Mechanicus will not like hearing of the ship loss, but they cannot stop the Rogue Trader from acquiring or even building another vessel. The number of available Navigators however is limited, while Navigator Houses have many clients to serve. Would they risk putting 4 navigators on a single vessel which might simply be destroyed by warp storms or aliens etc? Would they risk making 3 members of their house more or less disposable? Or would they rather provide a single navigator, so the captain is forced to protect him with every means available, while the others can serve other Rogue Traders who in turn become "vassals" of the Navigator House? Furthermore, it's not the RT's ship that earns the long term profit of the Navigator House, it's the shares in the trade routes and colonies he established for his house. These will endure the captain's death and the destruction of the ship.

During my last Rogue Trader campaign I decided that the PC's ship only had one Navigator (a player character) and one Astropath (another player character) and decided to leave it at that, knowing full well what that could mean if either character died. I did this because a ship parsecs from civilization with one or both of those positions vacant due to PC death was much more interesting than the alternative. The death of any PC should matter, but the death of PCs that vital really matters, and I like the stakes to be high whenever the PCs leave the ship. All this was explained to the players upfront, but I did offer them a backup plan. During character creation I allowed them to purchase auxiliary Navigators and Astropaths by sacrificing 1 Profit Factor per replacement, with the additional option that while in port they could spend additional Profit Factor to increase the Rank of the replacements. The players chose profit over preparedness, and a good game was had by all.

Navigators: Two, of which one is master and the other an apprentice. The idea being, the master charts the general course through the warp and makes the actual decisions that matter, while the apprentice keeps watch. Both do 12-hour (or 1 1/2 -segment) watches. They get plenty of spare time when the ship is in realspace, which it will be for at least two weeks every time the ship goes to a planet (a week from system perimeter to planet, a week back).

Navigators are rare and expensive, no point in having more than necessary. Having two lets the ship run all the time, and should one die, the other one can still limp you back to port, albeit slowly.

As for astropaths, you wouldn't need many shifts of them, since you don't need to send messages 24/7. One astropath transcendent (rating 2 as for starting character) and a choir of 5 helpers is what I deem to be standard. If you have an urgent message to send during the "night" shift for them, you just wake them up.

I suppose I'd have to go against some of what has been said and say, the chances of a Rogue Trader - the premier ambassador of the Imperium of Man, beyond the realm of the Imperium of Man is never going to be in a position where he lacks for navigators or astropaths. How are the realms of the Imperium expected to expand and reclaim their lost territory if a Rogue Trader with a fabulous warrant of trade can't perform his duties - because of a manpower issues? I think it's ridiculous.

I won't argue about a minor RT with a less-than-player worthy Warrant or a House that has fallen and been disgraced - but the warrant a typical player will have access to, should be taking for granted their access to Navigators and Astropaths. It's in the best interest of the Imperium to keep a RT in good standing, supplied with the basic necessities for travel and exploration beyond the borders.

Astropaths are expendable tools that an Astropath Transcendent, with the permission of his RT can burn through if a particular transmission requires it. If their were any agency of the Imperium that might detect xeno or heretic forces gathering beyond the Imperium it's a Rogue Trader that would find it. If anyone is going find some useful tech, beyond the Explorator Fleets (beholden to the Machine Cult first, and the Imperium second), it will be a Rogue Trader. It does no one good, if these rare and famed explorers don't have ready access to Navigators.

In my opinion the Rogue Trader houses, have pacts and terms of service with one or many Navigators Houses - through ancient treaty, or Imperial decree that implore them to supply them as needed - for Rogue Traders, and especially Rogue Traders with powerful Warrants of exploration.

I would also disagree that a RT would only have a single Astropath Transcend, as the Transcend are most valuable beyond and at the edges of the Astronomicon, which is where the greatest need and demand for them is required.

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I think it's pretty reasonable for a Rogue Trader to keep a number of astropaths kept in cryo, and thawed out when they burn through them - since being beyond the Astronomicon is going to drive them insane eventually. Now, if a Rogue Trader is especially gregarious about expending them, then by all means, have them require a test to acquire more of them.

On the other side of the coin, I would also warrant that the Navigator Houses use Rogue Traders as training grounds for their most promising brethren, all under the tutelage of a primary navigator who's role is to teach - and under ideal circumstances,another navigator who answers directly to the Rogue Trader. All of which participate during a warp transit. And again, I would assume it would be good practice to keep some cryo'd as well.

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I totally agree that some Captains and Traders do indeed have issues with Navigators - and are at their mercy moreso than a player Rogue Trader. Some vessels may only have a single Astropath who is only a passive receptor of the most powerful transmissions, but not capable of responding - while some might entirely go without.

I have always had an idea in mind that if the Navigator was killed then my pc's characters wouldn't be lost but would likely have to make a deal with a more unstable (possibly daemonic, warp witch) psyker, they would have to seek out. Just a thought.