There is not a limit to the number of subscriptions you can add to your FLEET account. You can keep a pool of subscriptions handy for easy enrollments.
NOTE: Subscriptions do expire if they are not applied to a generator within two years (from the date of purchase of the subscription). After the expiration of a subscription, it will no longer be available to be added to a generator. Subscriptions are used in a FIFO (first in and first out basis).