4 in. White Recessed Lighting Kit

Yes, you can install multiple units on the same circuit.

With a compatible dimmer, yes.

Yes. But only if you use an LED bulb that is 7w or less.

No. Incandescent bulbs emit a lot of heat which could cause a fire. The only way incandescent bulbs could be used is if you move the insulation 4" or more from the light. Stick with LED. They're much cooler.

Yes but it’s cheaper to buy it but the four pack

These will work nicely in a bathroom, Home Depot carries a wide veriety of shower trims. Remember any fixture in the shower must be ground fault protected.

To install in a drop ceiling you would need a standard can, this style is for remodel only. As it uses the drywall or paneling for support. Ceiling tiles might be a bit flimsy.

Yes you can find a LED version of this lamp.

We did one light about every 5 ft. for a family room that is roughly 13x 23. We put regular LED bulbs in them instead of the flood bulbs though because I don't like how glaring flood bulbs can be sticking out. We also put four in living room that is about 12x15 (one in each corner set in about 3 ft). I like how they both turned out. You'll just have to look at your space and see what you think. If it's a basement, put more; if you have lots of windows, you can probably put fewer.

Non-IC; instructions say it requires a minimum 3 inch clearance from insulation.