Power Editor currently has limits on the number of ads and accounts you can manage:
A user can manage up to 25 ad accounts.
An ad account can have a max of 25 users per account.
A regular ad account can have up to 5,000 ads that aren't deleted.
A regular ad account can have up to 1,000 sets that aren't deleted.
A regular ad account can have up to 1,000 campaigns that aren't deleted.
An ad account can have up to 50 ad groups that aren't deleted per ad set.
You can read more here: https://www.facebook.com/business/help/652738434773716