If you're doing the yogurt raw, I highly recommend starting with either powder or pasteurized yogurt. Raw yogurt culture doesn't survive generations very well. I've kept home-pasteurized yogurt going 8 or 10 generations before I slacked off and had to start over some time later, it never went funky on me. When I started making raw yogurt, a local raw milk farmer, who makes fantabulous award winning raw cheeses as well as raw yogurt, told me I wouldn't be able to do that with keeping my milk raw. He was right - generation zero (the one started with the powder) is great. Generation one is fine. Generation two invariably is gross. So, I make a quart with the Yogourmet powder. I divide that quart between four new quart jars and make the first gen yogurt in each of those; thus, I end up with a gallon of yogurt for each packet of Yogourmet starter.
Now, if I were really organized, I would keep a small batch of yogurt going with home-pasteurized milk, so I could always provide my own starter for the raw stuff. But, I just ain't there