How many fielders are allowed on offside in cricket?

The only restriction in the Laws of Cricket is that in Law 28.4:

At the instant of the bowler’s delivery there shall not be more than two fielders, other than the wicket-keeper, behind the popping crease on the on side.

This means you could have 9 fielders on the on side if you wish, so long as 7 of them are in front of the popping crease, or if you wish, all 9 fielders on the off side, either in front of or behind the popping crease.

Individual formats and tournaments may introduce further restrictions; for example, the restriction to having no more than 5 fielders on the on side is part of the Playing Conditions for both One Day Internationals and T20 Internationals:

28.4.1 At the instant of delivery, there may not be more than 5 fielders on the leg side.

but that is not part of the Laws of Cricket. I don't believe that any ICC tournaments have a restriction on the number of off side fielders, but you would need to refer to the Playing Conditions for the specific tournament you are interested in if you want a definitive answer.