To echo what @bdemers said, for sure! We specifically built the free tier so people could use it for small projects and personal apps. A number of us use it for our own projects.
As long as you don’t need any of the features in the paid or enterprise tiers (email customization, SAML, SLAs), you’re totally fine using a free account for <7k users. If you ever change your mind or decide to upgrade, you can do so straight from the Developer Console by clicking the Upgrade button.
What are the differences if one selects the Upgrade button and the $50/month option? It seems that the free developer plan accommodates 7000 users - does the option identified above lower the number of users supported to 2500? As well, if one selects the $50/month option, how does the support level differ?
@jwf While I believe this information is correct for new accounts, those users that have migrated from Stormpath (such as myself) to the free Okta developer account during the transition period may have 7000 monthly active users.