As long as the impedance stays high enough that it's at a level the amplifier can safely drive; no, there is no limit. However, the more speakers you use, the less power each individual speaker will receive. And with the crappy HU outputs, you can imagine that ~15w RMS won't go very far if you want to split it between multiple speakers.
im really asking this for my friedn, the one with all the money......he wanted to hook a bnch of tweeters up...i thought it would look tight but im not so sure how he's gna do ti, so im inquiring about how i was thinkin of doin it....
edit: and apprantly his hu output is 45x4max not sure what that is rms but probably near ur 15.. he has some pioneer hu