So just doing the basic math here, but the minimum number of accounts here is 3.
Five level 50s, and one of those is on an account that hasn't been locked.
Four of them are on accounts (plural, but unspecified number) that have been locked. Because OP said accounts, that means more than one was locked, and yes there could have been five total, but those four characters could be spread out on two accounts with 2 on each or 1 and 3 split between two. Could also be 1, 1, and 2 on 3 locked accounts, as well as a total of 5 accounts with one level 50 on each.