The answer to both this and /u/The_Manchurian 's question is that we don't know because we don't have sufficient reliable primary sources to reckon any such number.

The two basically reliable, contemporary documentary Islamic sources we have are the Quran and the constitution of Medina. Pretty much everything else that we know comes from one of three sources: the tafsir (exegesis of the Quran), the Hadith (reported sayings by or about the prophet) and the Sira/Maghazi literature detailing the prophets life and military campaigns.

The problem is that these latter three sources didn't get written down until a couple hundred years later. So while there is some rather intense historiological debate about their overall reliability, I don't think anyone would defend them for something as specific as casualty figures. We basically know that Muhammad organized military campaigns, organized the community of believers into some kind of religio-tribal confederation or quasi-state structure and that's about it.

Something I was thinking of before, that I'd like to ask as a subsidiary question: I recently was reading about the Banu Qurayza, who apparently betrayed Muhammed, were attacked in retaliation, and were defeated in battle, followed by their (apparently) execution (well, all of the adult men, about 600-900) at the order of one of Muhammed's followers (with his approval). Here's the Wikipedia article: https://en.wikipedia.org/wiki/Invasion_of_Banu_Qurayza

Anyway, it all seems a bit unclear, where does this information actually come from? How reliable is it that this happened? A Quranic verse is mentioned, but it's rather vague, and I have no idea who the Islamic jurists who described it are (were they contempories? Were they writing hundreds of years later)?

So, any illumination would be appreciated.