You can't really OD on bacteria!! But you don't need to go overboard - just the fact that you're having any extra will benefit. When your gut is functioning properly, the healthy bacteria that are there naturally will reproduce as they should. This keeps the levels appropriate naturally, too. A bit of a top-up here and there is always good, but you'll never actually be without!!
A healthy gut should contain something like two pounds of bacteria, and considering they're microscopic, that's a humongous amount. To answer your Q though, it's be OK to have more than one yakult a day, and even to have tablets alongside, but not necessarily more beneficial. Definitely more expensive though...!
Personally, I stick to a little live yogurt every day, and rely on the bacteria all reproducing like crazy lol. (Oh, and keep your tablets in the fridge too.)
:o)