If you would like to have more decompilers than your plan offers, you will be able to upgrade to a higher plan. For example, if you choose the Expert 2 plan with two decompilers of your choice, and in the middle of your subscription period, you want 1 or 2 additional decompilers, will be able to upgrade to Expert 4.
Decreasing the number of decompilers in the middle of a subscription period is not possible.