I'm pretty sure if someone asks you how many chips you have left, you don't have to give a verbal answer, but you have to make sure your chips are not obscured so s/he can count for himself. I also think that the dealer can count your stack for the person asking. Another play shouldn't touch your chips, nor should you ever touch his.
I have no idea what tells are given off by the response to such questions, except I think that good players don't care what you say--any reaction to their question will give them information. Just be careful of the strong means weak response if you ever choose to answer someone's question. (See
Caro's website for explanation of this.)