Yes you do.
Have you seen a cheap e-bike that is single speed and think, “it has a motor so why do I need gears?” It is a fair assumption, however, in most cases it is a little inaccurate.
Pedal assistance e-bikes are just that, they are limited in power, and require the rider to provide some input. If you are simply riding on flat ground, for short distances at a constant speed, then in theory a single speed e-bike is fine. In practice however, there are few applications that it would work well in.
If you have any sort of hill, carrying luggage, encounter any wind resistance, change in riding surface, or want to alter your speed then you simply will need a range of gears. Riding a single speed bike in any of the previously mentioned environments is going to overload the motor, and potentially cause damage to the e-bike system components including the motor, controller and battery. It is likely that you would still need to walk the bike up some hills, and also not be able to pedal fast enough to stay at 25km/hr, so it really doesn’t satisfy many requirements.
How many gears you need, really depends. Most commuting e-bikes tend not to be ridden above the 25km/hr and so the working range is 5-25km/hr, assuming that there aren’t too many hills, a 7 speed drivetrain can be suitable. For mountain biking however with massive changes in gradient and speed, a larger range of gears is required.