We know 9 shots were fired, but considering the inaccuracy of handguns, it's safe to assume he was not actually hit 9 times. Very few police ever discharge their weapons - some go a whole career without doing it while on duty - and they tend to miss even at close range.
That said - 9 does seem excessive and it does seem like the option of tasering the kid was available to them. They don't train for leg or arm wounds to put someone down, which makes you think that more training and a wider range of options need to be available. I kid with a knife, trapped on a streecar by police, should not be shot unless he actually charges the officers. I watched the video, and it's clear the streetcar was emptied and he was on there alone... they could have sat and waited the kid out.
I'll bet only 1 of the first 3 shots was an actual hit and it seems even the follow up shots (or misses) didn't initially cause him to release the knife, hence the tasering.
A clusterfuck by any reasonable assessment. I feel sorry for the kid's family.