How to separate milk kefir grains?
From this comment on a passionate homemaking article, the commenter suggests that 2 Tbsp per quart of milk is an appropriate amount of kefir grains. In my personal experience, I've found that the amount is fairly variable, and that half to twice that suggested amount will produce kefir relatively quickly (how quickly, of course, changes with the amount present). 

As far as separating them goes, they should pull apart relatively easily with the use of a couple of plastic spoons. While it is possible to smash them apart, this won't actually damage their ability to ferment kefir. My recommendation is to keep a 2 Tbsp measuring cup sanitized the next time you change out your milk, and add them to the fresh milk once there are enough to fill up the cup. The remainder can be used on another batch, eaten, or put in a sealed jar in the fridge with a smaller amount of milk to serve as a backup for later. Your worst-case here is that you just throw away the excess, which I would only consider "bad" if you can't find someone to give them to.