Its possible to wire as many as the adapter provides current for.
Its common to see 12v 500mA adapters, and its common for a computer fan to draw 50mA @ 12v. In this example, that adapter could safely power 10 fans.
Read what the adapter current says, then look what the fans draw for current and make sure you dont exceed what the adapter can provide.
Just for reference,
1 amp = 1000mA
.001 = 1mA
.050 = 50mA etc. etc.