I've been told about 500 gallons per big cat, sounds about right to me.. but my cats are still pretty small so I won't know 100% for another years yet. I'de guess 3-5 as well depending on filtration and personalities. and if your keeping RTCxTSN hybrids stick with RTC's, hybrids, TSNs, maybe marbled pims.. but the "heavier" set ones. It's not uncommon for big cats to cannibalize either so make sure everyone is as close to the same size as possible. IE grow-out tanks ect. I've seen plenty of pictures of big RTCs with smaller ones in their mouthes and the hybrids are well known for this. imo more then a 4-6" difference at 24"+ and your asking for canni behavior.