What’s the most I could be billed in a month?

Your maximum includes the plan fee, fixed add-on-seat charges, and any overflow already incurred; pay-as-you-go also includes your remaining cap headroom.

With cap-and-hold, the figure is your plan fee plus any fixed add-on-seat charges and any overflow already incurred this month. With pay-as-you-go, it also includes whatever remains under the overflow cap you set. Your dashboard shows this “max possible bill” figure directly. (Usage-based build/compute, if you use it, draws your prepaid balance separately and isn’t part of that number.)