CBSubs currently doesn't have quantity management. You can however disable a plan using the below database query as part of the CBSubs SQL Actions "Activation" parameter. This would set the merchandise plan to no longer accept new subscriptions after a number of purchases. It's not a easy to use parameter, which would be more ideal, but it'll certainly do the job.
Code:
UPDATE `#__cbsubs_plans` SET `allow_newsubscriptions` = 0 WHERE `id` = [plan_id] AND ( SELECT COUNT(*) FROM `#__cbsubs_merchandises` WHERE `plan_id` = [plan_id] AND `status` = 'A' ) > QUANTITY
With the above simply replace QUANTITY with the number of active purchases you want to limit it to. For example set it to 5 and after 5 active purchases of the merchandise plan it will set the plan to no longer accept subscriptions so no one else can further purchase it.
krileon
And I do hope this gets in as a feature in CBSubs, so it can be used more easily for Events Registration, etc... Unless you or someone else have a better way of doing it with CBSubs