First, open the edit setting for your hub.
Under the new "subscriptions" tab you will find the setting to limit your SMS subscribers. This feature will be disabled by default.
To enable this feature click on the "don't limit" switcher and enter the number of subscribers you want to allow to receive updates via SMS.
Then hit "Update" to save your changes.
*** You should be aware that the number of subscribers will not limit your quota - e.g looking at the basic plan if you placed a limit of 100 SMS subscribers and each subscriber received 5 SMS's each you would still reach your quota.