The maximum number of Boosts (monthly optimizations) you can have per month is limited to your subscription plan. It is not possible to keep your unused Boosts (monthly optimizations) for future use.

An example of what would happen to your Boosts (monthly optimizations) on your next billing cycle date:

Boost plan (1,3 or 5 channels) (no rollover)
You started the month with 40 Boosts. You used 10. The leftover Boosts expire. The next month, you start fresh with 40 again.