The Membership pro Reminder plugin has a parameter for Number Subscribers ("the number of subscribers which we will send reminder each time the plugin run") . What does this mean? How often is the plugin run? The value is 10 and I have about 250 subscribers.
2. You can set it to 10 or 20, but should not be a too large number. The reason is because most hosting provider will prevent you to send too many emails within one hour (they might limit to 150 per hours)
Don't worry about 250 subscribers. The system will continue sending reminders until all subscribers receive reminder emails