The only way to limit the plan to a number of purchases is using a combination of CB Query Field and the conditions under the Workflow tab. There's only functionality built in to control the number of renewals using the "Total number of occurrences of regular validity/price (0=unlimitted)" parameter.
But, How do I turn off renewals?
Edit the plan and under Pricing set "Renewable in advance by" to "Non-renewable" or use "Total number of occurrences of regular validity/price (0=unlimitted)" to limit the number of renewals (necessary if you want renewals to stop for recurring payments at a specific amount).
...and will any of these changes affect current members of a plan?
Should continue to work fine. They just won't be able to renew and new users won't be able to subscribe.