HOW MANY PRISONERS ESCAPED?????

In a jail there are 100 cells each containing 1 prisoner each numbered from 1-100. One evening the jailer came in drunk and openned all the cells, then he closed all the cells beginning from cell 2 whose number is a multiple of 2. then he went to all cells which are multiple of 3 and openned it if it was in closed state and closed it if it was in opened state, he continued doing this from 4-100. After having done this till 100th cell he left the jail. Assuming that no prisoner escaped in between this and all prisoners whose cell were in openned condition can escape, what is the maximum number of prisoners that escaped?