I know that you are only allowed to have eight units in a tile at the same time, but does that limit also apply to attacking armies?

For instance, am I only allowed to attack with eight units at a time, even though I can pull in reinforcements from other tiles?

You may attack with as many units as you want, but after the (won) battle, you have to retreat all units that exceed 8. If there is no area to retreat to, your units will be destroyed as usual.

What Graf said. Also, note that anytime you retreat, you must retreat all the units to the same area; you can't spread them out! (The only real exception is the Sorceress ability, because those retreats happen "separate" from the main retreat at the end of the battle, so you could move them to a different area).

In other words, if you bring in, say, 12 units, and have all 12 at the end, 4 must retreat, and they must all go to the same area. This is true even if every legal area would cause the new area to have more than 8; in that case, anything in excess of 8 would be destroyed. You could not split them up and retreat 2 to one area, and 2 to another.

Thus, in practice, if you bring more than 16 units into a battle, you are guaranteed to lose some units in some way. You'll probably win the battle, but only 8 can stay, and when retreating, the rest must move all at once.

(Also, one more note: Keep in mind when you retreat, you MUST move to a friendly area if there is one, even if doing so would cause you to go over your limit. The ONLY time you can retreat to an empty area is if there are no adjacent friendly areas at all.)

In other words, the game encourages a reasonable amount of overkill. An excessive amount will be slapped down. =P