Over the last few years, the MAX8 has been outselling the 320NEO because it has better CASM. This is due to it having a few more seats vs the 320NEO. The 320NEO has been rumored to grow by 10-20 seats. That would give it superior economics to the popular (not at the moment) MAX 8. This also makes space for the 220-500 business case. Either way, a -500 won't come until the A220 supply line matures. Could be a while, or never. But when the CEO says "In the future Airbus is going to want to bring on the 500..", it's not just unsubstantiated fan speak.
So you have potentially by seats :
110-120 =A220-100
130-140 =A220-300
150-160 =A220-500
170-180 =320NEO+
190-200 =321NEO
210-230 =??NMA XLR/322