A few habits that prevent sprawl:
Archive boards that haven't been touched in 90+ days instead of deleting them
Archived boards can be restored later if needed
Do a quarterly pass to merge duplicate or near-duplicate boards
Review workspace membership when people change teams or leave
Checkpoint
Your workspace should have folders that map to real teams or projects, at least one board with tightened permissions, and a plan (even informal) for a periodic cleanup pass.