There's no customer limit parameter for plans. Only way I can think of to do this is probably conditions with help of CB Query Field or use CBSubs SQL Actions to change the plans parameters so it doesn't allow new purchases once a number of users have subscribed using SQL. The below for example should work.
SQL Action 1
Activation:
Code:
UPDATE `#__cbsubs_plans` AS p SET p.`allow_newsubscriptions` = 0, p.`allow_registration` = 0, p.`allow_upgrade_to_this` = 0 WHERE p.`id` = '[plan_id]' AND ( SELECT COUNT(*) FROM `#__cbsubs_subscriptions` AS s WHERE s.`plan_id` = p.`id` ) >= 100
That should turn off "Allow new subscriptions" under the Access tab and turn off "Allow registration" and "Allow upgrade to this" under the Workflows tab if the plan has 100 or more subscribers. It does not check if they are active subscribers. It's just checking if a subscription row exists for the plan. Adjust the 100 in the query to whatever you like.