From SR experience I believe we should stay away from more than 14 teams. In fact, it'd be better to go to 12. That for me is the magic number. It means all teams can play each other and its as likely to be strength vs strength as we're probably going to get. It also just becomes too many teams and casual fans probably can't be bothered to keep track of which teams is which and where from if more than 12 especially if the teams don't have any clout.
So IMO stay away from more than 12 teams and avoid the conference system. The conference system hurt SR as much as the actual # of teams maybe more than the #.
SA would love to have more than just 2 teams though. We have 5 very viable teams with a history of high level involvement, the Kings are a political must (and if the backroom could get sorted *could* become a powerhouse in their own right). Then there are the Griquas and Pumas who punch way above their weight. I suggest the Currie Cup of 8 teams be used as a promo-relegation tournament for entry into Pro1X with the bottom 4 SA teams entering a 2nd tier competition running parralel to Pro1X. In the same breath I'd say cull two teams from the old ProX both to keep that competition a premier one and in order to add numbers to the 2nd tier and make that one also viable. I don't know enough about the teams to comment yet but I've heard tell Wales cannot feasibly field 4 quality teams and Italy doesn't seem overly hot. So maybe something along the lines of:
Pro12:
Ireland top 4, SA top 4, Scotland top 2 & Wales top 2 = 12
My prelim rankings (1 Leinster, 2 Sharks, 3 Stormers, 4 Munster, 5 Bulls, 6 Edinburg, 7 Glasgow, 8 Ulster, 9 Cheetahs, 10 Connaght, 11 Ospreys, 12 Cardiff)
Not-quite-as-pro (Por?) 12:
Italy 2, Wales 2, SA bottom 4 (probably currently the Lions, Pumas, Griquas and Kings) = 8
This also allows room for further expansion without diluting the premier comp. Do Scotland and Italy want to develop more teams in the near future? If not, complete the roster of 12 with invitational sides. With SA there maybe it makes sense for Namibia, Zimbabwe, the Netherlands or Belgium to field a development(para-national) side each like what Argentina did with the Jaguares in the Currie Cup before they joined SR. Germany, Portugal, Spain? Speaking of Argentina, aren't they also now out in the cold? The Jaguares were muchos competitive towards the end.