You can edit your plan and under Pricing tab set "Total number of occurrences of regular validity/price (0=unlimitted)" to 50 which will limit the maximum amount of purchases for the plan in total to 50, but as I understand it you want it to only be limited to 50 active users.
To limit it to 50 active users you'll need a CB Query Field to query the _cbsubs_subscriptions database table and query for the count of active subscribers to that plan. Then under the Workflows of your tab you'll need to setup a condition against the Query field (you'll likely need to use "Constant Value or String (CB substitutions can be used)" and substitute in the query field) to ensure it's less than 50. Example query as follows.
Code:
SELECT COUNT(*) FROM `#__cbsubs_subscriptions` WHERE `plan_id` = PLAN_ID_HERE AND `status` = 'A'