Is there any precedent of group conversion in judaism?
Is it acceptable to hold a conversion process where 60 people are converted?
There is no halachic precedent of group conversion, though some claim that this was practiced in the past, for instance in the kingdom of Kuzar [some also claim that it was done in times of the Hasmoneans].
Conversion is essentially a personal matter. If out of a certain group, particular individuals are not wholehearted, and they do not wish to covert, their superficial conversion would not (according to most halachic authorities) be valid. It depends on the individual rather than on the group. If all are wholehearted, then the fact of their forming a group does not affect their conversion.