number of yoyos you should have

No need to limit. Keep buying more, haha

A first yoyo. For beginners.
A second yoyo, (If your first was like, a Duncan Butterfly) to start getting those tricks down.
A third, possibly unresponsive yoyo that can take you to Intermediate or Advanced
A couple 2a, 4a throws if you wanna branch out.
A fourth, unresponsive beast to step up to the next level.
A job.
Lots of metals.