It’s deposit volume rather than number of accounts that are over $250k. I’m sure that most of the accounts are <$250k, but a single $1m account accounts weighs the same as 100 $10k accounts.

But why is anyone holding $1M in an account insured up to $250K?

Many companies pay out their workers more than $250k per pay period.

If you have 100+ employees for payroll.