MSP will notify you when you reach the limits of your subscription and prompt you to upgrade.
Until you upgrade, you will not be able to add more volunteers to the program, but everything else will work the same as usual. If you do find yourself with more volunteers than the program allows, you will not be able to create a schedule, export volunteers, and some of the other features in the program will have limited use.
At any time, you can log in to your account and click "Upgrade" to add more volunteers, installations, or upgrade to the Plus Edition. After selecting your options, click "Continue" and you will be given a price breakdown, and can choose "Continue" again to be brought to the payment screen.