It took me a couple of searches, but I knew I saw someone else ask the shape in the morning question! 
answer 
The most I’ve made at once is 18, in two batches of 9. I was concerned about boiling and topping all of them at once, mostly because the transfer from somewhere? to the boards of the second batch and how long they’d be sitting wet. (If you try that though, please share how it goes.)
Doing it in two stages worked well, but remember to re-soak your bagel boards. Last time, I forgot to do that until the last minute…my first three bagels were ready to be placed on the first board and they were all sitting across the kitchen, toasty hot and dry.
It wasn’t a big deal…just a 30-second pause.