Ive never made my own image board, ive only moderated for a different image board in the past.

But i would recommend if any site is starting, probably to start with as few boards as possible, like.. just 1, and then expanding those boards as users grow.
It would be easier to manage for you, and it will also feel more active when youre just starting.

Heyuri is a great site, and gets good traffic but even with the limited boards it has, its slow sometimes, so if your usership is less than heyuri's you should probably have much much fewer boards - for now.