No, monthly credits do not rollover to the next month. At the start of each billing cycle, any unused credits will expire, and your account will be refreshed with a new set of credits based on your subscription plan.
For example, if you're on the Pro plan with 40,000 credits per month and you only use 20,000 credits in one month, the remaining 20,000 credits will expire. When your new billing cycle starts, your credits will refresh, and you'll have 40,000 new credits to use for the next month.