How many devices can the unlimited plan support?

There is no fixed maximum. The Unlimited plan gives 24/7 access to the number of simultaneous virtual devices you purchase and you are charged per simultaneous device (e.g. $219/month per virtual device).

Supporting details: the docs state an organization “has 24/7 access to the number of virtual devices included in the plan” and that billing is based on the number of simultaneous virtual devices purchased — see the Pricing page and Cloud product pricing for per-device rates. Pricing | Genymotion Android Emulator in the Cloud (pricing)

How many concurrent devices do you expect to run (or what’s your team’s required test concurrency)?