Making my first big batch of Kefir, the milk went bad. Why?
I haven't made kefir myself, but this sounds very well explainable from a theoretical point of view. 

The reason why the milk in the kefir does not go bad is that the microorganisms in the kefir occupy the ecological niche which would normally be free for pathogens (and plain untasty bacteria) to invade. Obviously, you need a sufficient microorganism density for that. If the kefir colony is too small to change its environment and adapt it to its needs, a colony of something else can win the tug of war and adapt it to its own needs. 

I can't tell you an optimal ratio, you'll need to experiment a bit. Also, wash the salvaged grains and place them in the amount of milk which used to work. Only if that works, try gradually increasing the dose. If it doesn't work well, wait a bit to see if your colony is happy enough to grow, at which point you can try to up the milk. Or just get more grains from the seller.