How much seats would Ryanair squeeze in if they reach almost 200 in the MAX 8-200?
The MAX 10 is targeting certification for 230 seats, IIRC.
At the same seating density as their 197 seat 737-8200, they will be able to squeeze in 221 seats, maybe 223 seats. Absolute max would be 227 seats, but that would be even higher density for a few rows in the mid part of the cabin.
While the 737-8200 is used on the shortest flights, the Reuters article claims Ryanair wants to utilize the 737-10 on the longer flights to the Canary Islands etc. Revenue from on board sale (food, drinks and tax-free) is very important on those longer flights. The 737-8200 reaches 197 seats by eliminating the rear galley, except for trash disposal.
I find it more likely that galley space is preserved on a potensial Ryanair 737-10, and that they might also need to add a fourth lavatory on those longer runs. My best guess would be 217 seats. That seat count would retain standard rear galley, same as on their 189 seat 737-800, but not a fourth lavatory.
Maximum certified exit limit on the 737-8200 is 210 seats, and Boeing is aiming for that FAA/EASA will allow a 230 seats exit limit on the 737-10. This compares to an exit limit of 195 seats and 244 seats on the A320 and A321. European LCC is operating the A321 with 235 and 238 seats (Easyjet and Wizzair).
The 737-10 is about four meters longer than the 737-8, while the A321 is seven meters longer than the A320.