[Misja]

Asking how many hands we need for such a study is the wrong question. First we should ask what kind of checks we would like to do on the data. When we know that, then it can be calculated how many hands we need for those checks to give a statistically significant result.

For instance, some people say that they get sucked out too many times by people who go all-in. One possibility how a rigged site could try to accomplish that, is by filling up a possible straight - or flushdraw on the river in an all-in situation whereever that is possible and regardless of the hand of the all-in player.
This is easy to check and we would definitely not need millons of hands for that. A couple of hundred of relevant hands could already be enough to produce a significant result.

This was just an example, I am sure there are much more interesting checks to do.