Hi, quick question how many different weapons can one carry. Like my character, isn't it possible for me to have all my "standard gear" bolter/storm bolter, bolt pistol, ammo, 6 grenades, combat knife/chain sword and like a missile launcher or a 2h melee weapon at the same time if i take it as signature wargear. My gm pointed out its excessive but according to the weight carrying table i could carry alot more.

Typically most sane GM's uses the house rule of five hands.

Heavy weapons count as three hands used, Basic weapons two, and one handed / Pistol weapons as one.

Using that rule you'd have used four hands out of your five (Storm bolter is a basic weapon, so uses two, bolt pistol and chainsword uses one each so four total).

Ultimatively it is up to him to decide however.

While there aren't any hard and fast rules for how many pieces of equipment you can carry, common sense dictates that sheer carrying capacity isn't all that factors in. It's even more true in Deathwatch where the Astartes are so inhumanly strong, keeping track of carrying capacity at all is pretty much a moot point (unless one of the Battle-Brothers chooses to carry that combat bike on his back, or something like that). The really important factor is, how many weapons can you reasonably mag-lock onto your power armor in a way that'll allow you to easily reach them.

I'm inclined to agree with your GM here, your proposed loadout seems way excessive.

What if i strap my missile launcher on my back and ask one of my team mates to carry the extra missile mag.

This is how RPG-team act s IRL.

There is not enough space on your body to carry all that, unless you want to drop your bolter to the ground every time you want to use one hand.

As the book recommends, I go with:

1. One two-handed weapon (can be slung on back if need be)

2. Two one-handed weapons at waist (usually a melee weapon and pistol)

3. One combat knife at the boot.

The problem in your (OP's) example is your heavy weapon/2H sword. Everything else is reasonable, but a big weapon like that isn't really going to be 'slotted' onto a spot on your armor. In most cases those weapons are either 1: in your hands, or 2: on the ground.

For your other gear:

Bolter on a sling, you can just drop it and it stays on you.
Bolt Pistol in holster on right hip with 1 spare mag attached to holster side.
Combat knife directly forward of bolt pistol attached to holster.

6 to 8 Bolter magazines slotted on belt underneath backpack. (If it were me, no power armour and 5.56mm ammo they'd be in ammo pouches attached to my chest webbing, space marines don't get that option)

6 grenades, in 2: 3 grenade carry-clips attached to belt on left hip.

If you wanted to run a chainsword instead:
Chainsword with sling. Or scabard attached to belt behind bolt pistol and hanging below waist/backpack
Bolter with mag lock attached to forearm opposite of shooting hand.

Either way, no room really for a heavy weapon, except for your hands.