Does Genymotion access unlimited work on only one device?

No. Unlimited access is tied to the number of simultaneous virtual devices included in the plan, not a single device.

On the SaaS Unlimited plan, your organization gets 24/7 access to the number of virtual devices purchased, and billing is based on the number of simultaneous virtual devices, with the option to increase or decrease that count from your account settings. See Pricing and Find your perfect Genymotion plan.