At the time when morphing was invented I'm going to arbitrarily say 2342 died by Taxxon small pox, 434 by recreational plant wrestling and 9 whose deaths were hidden by the media actually suffocated on trying to drink liquidified Leerans.
In terms of how many Andalites died during experimentation with morphing technology I could make some numbers up like the above but chances are we'll never know. If I remember correctly the technology wasn't revealed to the public for quite sometime even the military didn't know about it (hence how Alloran was acquired in one of the books).
I'd like to think that the Andalites development of technology was overly cautious so safety first. It explains why they think humans were so quick to develop from the industrial revolution to spaceflight in such a short space of time. The only other reason is the lack of geniuses like Einstein which I'm going to simply ignore.