One use of switches is to protect your machine from overtravel that might damage the equipment. You would need two switches per axis-one located at either end. Each would be positioned to be tripped before the mechanical parts would bind or collide. These switches would be wired into the motor control circuitry to kill or stop the motor. These protect your investment and are for safety.
A separate use for switches is to mark a home position on your machine. Here you would have one switch per axis and it could be located anywhere between the saftey switches mentioned above. Your software would run a give axis till the home limit switch picked up. At that point it would stop the axis and consider itself in a know position, i.e. "0,0,0".
Thus for a three axis machine, nine switches minimum for a "complete" setup.
The six saftey switches really should be mechanical (as opposed to proximity, etc.) For the three home position switches I would probably use optical, as you want precise, repeatable switching. Others may have different opinions.
One more thought...please consider one or more "emergency stop" switches. These ought to be located around the work area where they can be reached quickly. Ideally these would be maintained position switches (push to trip, pull to reset) and would have large, visible red knobs. When one is pressed it shuts down all motors: it "kills" the machine. This is a safety item often overlooked.
Hi again. Well, I don't have much time this sitting, and I don't know the specifications of those motors. Can you list them?
In general though consider the following:
1. The newer steppers are much more powerful than those offered just a few years back. Thus you get more for your money.
2. A smaller motor (say a NEMA 23 frame size) might be just as powerful as a larger one (say a NEMA 34). Size alone is no good indicator: look at the torque curves. BUT the two motors will dissipate unused power as heat. And a smaller motor has less volume/mass, so, all else being equal, it will get hotter (not good). In the real world you need to look at the application. If the motors are not being pushed too hard, or the duty cycle (time used versus rest time) is low, or there is some cooling (forced air), then a smaller motor may be the better choice-especially if cheaper.
3. It seems that the better motor drivers are built for bipolar motors. With these you can use either four lead or eight lead motors. Six lead motors are called uniploar and need a different driver. Go bipolar in my opinion.
4. An eight lead motor, if set up as bipolar, can be wired two ways: series or parallel. Each has different speed vs torque characteristics. Current draw will also vary depending on the configuration chosen.
5. Get a motor whose current per phase is compatible with the driver you intend to use. For example, a Geckodrive (a very nice drive) has a maximum output of 7 Amperes per phase. So any motor less than 7 A would be a candidate, but one that pulled 9 A would not.
6. Your motors will run much better with a "chopper" type drive. The Geckodrive mentioned above is such a unit. The drives that require you to limit phase current with big power resistors will not perform as well.
7. Torque is your friend; more is good, less is not good.
8. You can increase torque, at the expense of top speed, with speed reduction. Many use toothed belts/pulleys. Cut the speed by two and you boost the torque by two, etc.
Hmmmm, I'm out of time and need to log off (I've several acres of mowing to do). Good luck!