How many bearded dragons should you buy?

you can have at least two or three and no more

Depends. If you have the bare minimum of a 40 gallon tank (for an adult), then only one. With a 75 gallon (55 gallons are too narrow for even one), you could comfortably fit one, uncomfortably two. If you have a male, watch him carefully even with females, and don't even let him see other males. For babies I would use something like a rubbermaid plastic tub for up to three or four beardies because its opaque sides reduce stress. This will last about until the beardies are about 15-20 weeks old, then move them to their permanent enclosure. For a couple or trio of beardies, I would recommend getting a custom cage or a very, very large glass tank from a wholesale company.