Really, you should be able to draw with as many people there are in the game. On the castles map, under placement/Att, barring an early rush, every player should be able to hold their castle pretty much indefinitely.
A good system of detecting how many people should be allowed to draw would involve analyzing the maps and finding how many regions of 8*n+c with only n entrances there are, or something like that. If you have any more troops than you need to fill your entrances to 8 each turn, you'll eventually saturate, if you aren't immediately wiped out. Once that happens, the game is, in fact, drawn.
Maybe checking to see if every player is saturated at the end of their turn, and the board state repeats itself a certain number of times would be a good check for the game to decide itself to be drawn.