The limit is 60 if you don't field a Warrior Priest or Arch Lector as they only count as Rare choices without one of them. If you field either of them, one, and only one, unit of flagellants count as a Core choice, leaving you able to take two more units of them as Rares in a game of less than 3,000 points. In a Grand Army of 3,000 points or more, you can field up to five units of them, or 150 flagellants.

Also remember that in any case you can only filed 60 (600 points worth) of them as rares if you are playing a game of at least 2400 points. You need a game of at least 4800 points in order to field 150 of them.