As many as you want. Paid Memberships Pro is 100% open source - there is no limit on how many sites you can use our plugin on. A PMPro License is required for automatic updates for our Standard and Plus Add Ons. A Standard license key can be used on 1 site, a Plus license key can be used on 2 sites. Contact us if you are interested in using a single license on more than 2 sites.