If you need more decompilers than your current plan includes, you can upgrade to a higher plan at any time through My Hex-Rays customer portal.
For example, if you're on the IDA PRO Expert 2 plan (with two decompilers of your choice) and later decide you want one or two more, you can easily upgrade to Expert 4 during your subscription period.
Decreasing the number of decompilers in the middle of a subscription period is not possible.