This is such an odd choice of pricing model. The usual approach is to say you get multi-device sync as part of a paid plan and single-device usage for free, or to place an upper limit on the device count, not count of types of devices.

My guess is that they want to limit functionality enough to make paid plans attractive, while still giving you the chance to try out how the sync works, but I can't help asking myself if this isn't unnecessarily confusing and going to put potential customers off as opposed to e.g. offering a 30-day trial on their paid plans.