Most top circuit teams have the majority of their roster spots filled before tryouts. But most also wouldn't hesitate to add a strong player if they are good enough.
And a lot of it comes down to whether you fill a need - an undersized guard/wing will have a tougher time finding a spot compared to a 6'+ post, for example.
My advice is to attend as many tryouts as you can fit in your schedule and afford, since some clubs do still charge a fair amount for HS tryouts.