Maximum amount of avatars

[Suzy Hoof 0]

Does anyone know what the maximum number of avatars you're allowed to have is?

[Rolig Loon 17,912]

According to the wiki, you are limited to 5 accounts per household.  Because a "household" might be as large as a college residence hall where several users all share the same router, it's not really clear how realistic it is for Linden Lab to enforce that limit.  In fact, it is widely rumored that the limit is not strictly enforced.  I suspect that the rumor is true for the most part, but that the limit would be suddenly enforced if you were to create an army of alts to attract the Lab's attention. 

[LittleMe Jewell 14,295]

It depends on the land -- per the SL Wiki

Maximum number of agents
Full region: 100 (Typically set to 40 on mainland, 55 on Linden Homes regions, but this does vary. Some meeting areas have this set to 60 and higher.)
Homestead: 20
Openspace: 10

[Cinnamon Mistwood 232]

Or, if you are talking about your own personal alt accounts, you can have 5 per email address.  Not sure which one you are asking about.