So there are a couple factors to milk.
1) It goes up every second based off of MPR (Milk Production Rate). You start with 2 and the max you can get is 4 in the demo, so that's 4 milk every second walking around. In battle, it's 10 milk per MPR per turn, so at 4 MPR, it's 40 per turn.
2) Max milk is based off level. Level 1 is 500 and goes up by 50 each level. So you got 900, that means you made it to level 9 :P