Likely several. Some out of a sense of right vs wrong. Some out of avarice with a possible big payout for the leverage these files hold. Some out of self preservation keeping leverage as protection. Lots of reasons for 1k agents to keep a copy.
It's hard to keep a secret when 1000 know the it.

Or one of the fbi agents assigned to work on it. They created a group spreadsheet to document whenever Trump was mentioned that all 1000 agents had access to. I’m sure at least one of them made a copy.

I just hope Jack Smith kept the original files and has them tucked away in some unnamed foreign country where the Trump fascist can't get to them.