Generally the shorter amount of time on each side of the vinyl, the louder and clearer the cut will be. In example, one 6-minute track on a side of vinyl will really sound much better than that track, plus another 5-minute track on the same side. You don't have to pack the grooves as much. Thats generally why a lot of labels are moving towards the new medium of 2 and 3 track releases instead of 4 these days. Genres like jungle have been doing this for quite sometime. I also really enjoy a full-sided record, so if the track is rockin' I would hope it has its own side.

I think the standard is/has slowly become 3 tracks, 1 on the A side, 2 on the B side. It's a nice way to meet in the middle between 2 and 4 track EPs. Other people will also argue its because techno producers are lazy these days and will only put out 2-track EPs to spread it over more releases and make more money. Thats an okay theory, but I think its more bullshit than truth (in most cases).

In my personal preference for putting out on my label as well others, I prefer 3 track EPs. More bang for your back, and it still sounds good.

As for deciding the actual number on each release, I am guessing it just varies from project to project. It all depends on how big you want the release to be (# of remixers, or how known the remixers are), how long the tracks are, how many tracks are available, etc.

Every label owner feels differently about certain releases so it's really hard to say what "seals the deal"