I took up Xerath first cause he was free, seemed interesting, and I thought I'd have a bit of fun trying someone new.
Once Sona became avalible I grabbed her just cause Xerath wasn't doing much and I always wanted her. Thought I could her justice like the last RP-er.
Of course, i could probably take on a third or even fourth champ easily, but then I'd deny someone else the chance to join in on us. I'd say 2 is the limit, (maybe 3, but only on special occations) so long as you can maintane those two. Of course, this is all based on a person by person basis. Some people can barely handle one, while others could probably take over all 108-ish champions professionally.
Besides, where's the fun if you take over all the champs? Then we don't get that champ to champ interaction this place was made for.