Questions about chemicals and reuse

I have a 1000ml bottle of DF96 that I use for my black and white. When I develop a roll I only use 240 ml at a time. If I pour in the used chemical back into to the larger bottle of un used chemical is that the same if I used the whole 1000ml?

Second question relates to tallying rolls processed. If I develop two rolls in one tank does that count as two or just one?