The number of magnets is simple. The wheel circumference is the distance travelled in one rotation of the wheel, so the outermost circumference of the tyre, not simply 2πr for a 16” wheel. This can either be measured with a flexible tape measure, or placing the wheel on the floor with the valve at the bottom and marking the contact point, rolling it for one rotation, placing a second mark at this point, then measuring between marks. Use meters for the units.