The main thing with number of devices they can handle is how much total wattage you have. Total watts have to be below the amount the stat can take.
As for different sources, you can run them with the same stat as long as wattage is met, but items of differing wattage will heat differently, so combining different things on one stat is discouraged.

If you can spare the cash, get the BEST you can afford now. I started with a Johnson, then had to add 5 ZooMeds and a Ranco! The Johnson is better than the Ranco IMHO because it's smaller, quieter, and easier to adjust. Now I wish I could have just bought/been given those Zoomeds and saved up what I spent on the Johnson and Ranco to get a Herpstat II...which is what I want now but cannot afford.

What that rant was trying to say is, if you see yourself getting more snakes in the future, then get at least the Herpstat if not a II or even a Pro now. It will be worth it not to have to buy extra stats later.