Slitherine

Is there a point limit or % of army that can be allies?

i.e. if I have a 650 point army, could 300 points of it be from allies?

The simple answer is that there are no points based limits on the size of your allied contingent(s), but you need to ensure that you've got all of the compulsory troops from your main army and that you abide by the restrictions on both lists. These are generally laid out in bullet points just before the main list starts but also apply to contingencies taken from the ally section. Some ally options represent specific campaigns or battles and have more specific restrictions, but generally you don't have a limit on the proportion of allies.

There is one practical limitation, namely that you can only have one general for the whole of your allied contingent (although if you have more than one allied contingent each one can have a single general). This means that you need to think about how many units he can realistically control in terms of "group moves" (especially if the contingent is a mix of mounted and foot, as these cannot "group move" together), stiffening resolve (ie keeping with, or within range of, a unit for cohesion and complex manoeuvre tests), and participating in melees. If he gets killed, that contingent has no more command support, as none of the generals from the main element - not even a GC - or a different allied contingent, can take over.