For a cassette-equipped direct-drive trainer, matching the number of gears is only the start. The cassette’s tooth range, drivetrain system and freehub interface must also be compatible. Depending on the package, another cassette, spacer or freehub body may be needed.
Rear-axle fit is a separate check. For example, the Tacx NEO 3M manual covers 142 mm and 148 mm thru-axle setups with the appropriate end caps; the 148 mm setup also needs a different support piece. The rear wheel comes off and the frame mounts on the trainer. These instructions are model-specific.
Before ordering, gather your bicycle’s model, year, axle dimensions and cassette designation. Compare them with the compatibility chart for the exact trainer generation, and include required adapters in the shopping list. If two bicycles will share the trainer, check each separately: this helps avoid awkward changes before every session.