However many you want, I would guess.

I know it's an unhelpful answer, but I found him rather easier than Moder and Bonemass when I fought him before Loxen would fight for you. (They were tameable, but not rideable.)

I'm not great at combat, either. I'm not exactly a dunce, but I'd grade myself poor to fair. Yagluth's attacks are very slow and telegraphed, and it's fairy easy to see what's coming.

I'm also unsure of how the Loxen would handle when he calls fiery death from above (how well they would survive).