Unfortunately, no, but it can be accomplished with an SQL Action. Supply the below to the activation query for your plan using CBSubs SQL Actions and it'll disable it after the plan has been subscribed 500 times. Please be sure to test thoroughly.
Code:
UPDATE `#__cbsubs_plans` SET `allow_newsubscriptions` = 0 WHERE `id` = '[plan_id]' AND ( SELECT COUNT(*) FROM `#__cbsubs_subscriptions` WHERE `plan_id` = '[plan_id]' AND `status` = 'A' ) >= 500
Note the above only counts active subscriptions.