On three of them.
If you open a subscription-only book or audiobook on a fourth device, the tablet or smartphone you first used to sign up for Bookmate will automatically be removed from the list of devices with permission to access it.
To allow and disable access independently, go to the “My device” section in your profile settings.
These restrictions do not apply to free books, audiobooks, podcasts, and books you have uploaded yourself. These books can be read or listened to on any number of smartphones or tablets.