All the comments below are based on motor control decoders without sound.
The decoders are rated with how many amps they can manage continously and short term.
How many motors depends on the amperage of each motor. So the question cannot be answered accurately without that information. What is the motor current rating (amps).
An N scale motor is generally very small, and draws very little current. Therefore a decoder with small low current components can be used and the decoder can be built very small. And HO motor is bigger and generally draws more current and needs a decoder with a higher rating which usually means bigger. An O scale loco usually has even larger motors and draws more current and therefore needs a higher rated decoder that is generally larger and more expensive.
Technically, for motor control, there is no difference between HO, N, and even O decoders except as mentioned on what kind of current and sometimes voltage they can handle. There used to be very low end ones that only used 14 speed steps for control, but I don't think any of those are made any more. I think most you find today will be 28/128 speed steps for control and the decoder decides this automatically. There are differences in the auxilliary outputs from the decoders for lights and other accessories. There are very cheap ones that only have the motor control, and 2 functions - one for front light and one for rear light. The more expensive ones usually 5-6 outputs or more for all sorts of lights and whatever you want based on the amperage rating again.
NCE makes a great motor control only for O scale, model D408SR. this decoder has a 4 amp continous rating and a 10 or 12 amp (I can't remember) stall rating. If your motors draw one amp apiece under load, this decoder would do the job. It is about 1x2x1/4 inch, so is quite large but very capable for control. I think it has 6 aux outputs.
