Your downlights need to be suitable for the environment. So at least IP44, maybe IP66 depending on where in the bathroom they are being sited.
They must comply with building regs part C2 (Resistance to moisture) otherwise teh steam will get pulled into your roof space, where the damp will rot the timbers
Now, how much light do you need? How big is your room, what needs light, and what doesn't. Do you want to dim them, or not?
The amount of light delivered will depend on the output of the lamp (now quoted in Lumens), the angle of the beam (these vary greatly), the height of the ceiling to the surface to be illuminated and much more.
Also, there are several light colour types, white, daylight white, warm white, etc...
Have a read of this Photometric report on a typical downlight, and you'll see there's a lot to think about
http://www.tlc-direct.co.uk/Technical/DataSheets/LEDlite/LTFRD12DL.pdf
You can buy them from any reputable electrical wholesaler, or many on-line suppliers.