How many times can I apply for an evaluation license?

For server and data center apps, you can extend your app trial up to 5 times - in other words, for up to six months. Extend your trial by generating a new evaluation license key from Atlassian Marketplace. Click Try it free and you'll be directed to generate a new license. Paste this license key into the app listing in UPM from your Atlassian host product, and you're all set.

See Atlassian's Pricing FAQ > Can I extend my free trial? on the page bottom, here: https://marketplace.atlassian.com/apps/1213072/repeating-issues?tab=pricing