One per side. I used a cheap single cell holder from Amazon which isn't ideal because my recycled cells had solder tabs so the top is recessed slightly and doesn't make good contact with the holder. I added a blob of solder to build it up, as I remember. I mean, it works ok, but it needs to be better. I was going to buy the holder contacts and print my own holders, maybe see if I could get two cells per side (probably not enough room), but got side-tracked. The 18650 is an easy hack, though, and doesn't require any modification to the handle light if you want to do it. The thing I worry most about are those stupid alkaline batteries leaking all over the inside of the handle light and damaging it...I can put up with shorter battery life to avoid that scenario.
I can take it off the grill and post pictures if you'd like...not elegant, but it works...I don't know how well it will work in cold weather, either. At some point a cheap $4 charger module could be easily added to charge the cells in place, too.