How many walk-ons do you have to have before it negatively affects your practice growth? I heard it's either 2 or 3, but I'd like to be sure for the future.

It's 2. But I wouldn't really worry about it. A very minimal effect.

Im pretty sure its 3 because thats the point where you can no longer run full 5 v 5 scrimmages.

i honestly cant remember 100% for sure... might be the alcohol among other things... i think it was that 1 walkon = 0 penalty, 2 walkons = trivial neglible penalty, and 3 walkons = some possibly measurable practice penalty. in reality, 4 walkons does not cause major damage in practice planning. 

i would highly recommend totally ignoring walkon penalties, and play accordingly. if you think it would help you, outside of practice penalties, to take 4 walkons - then by all means, do it.

I've run several times with multiple walk-ons and the way the game is set up there really isn't much of a penalty at all.  Reason being is that the extra playing time your guys get pretty much negates any practice penalty.  If it happens that you have multiple walk-ons for a season, don't sweat it at all, it's not the end of the world.  In fact, you'll barely notice the difference.

Officially, it's 2, but we have been told that 2 is only a small penalty, and that it increases at 3.