This granny square blocker is currently holding a stack of 15 squares in worsted weight, and I could easily fit 3-5 more squares on top if I wanted.
The board and pins are sturdy and fit securely together. I never have to fiddle with the pins after placing my squares.
A word of caution if (like me) you're making squares larger than 7 inches: the working area is 8 x 7 inches, NOT 8 inches square. All that means, though, is that you'll have to get creative with your pin placement.
I'm super glad I bought this blocker and am excited to use it again and again!