like mentioned probably 20 or so, but start out smaller because they'll breed easily and youll be flooding shortly after.
you'll need posting hitches,fake or real(most use fake)....your tank must be sterile so you dont get things like hydroids,etc. Panacur is a medicine you should have as it will help take care of these things, and anything live should be treated before putting in the DT....you'll of course need to feed baby brine shrimp daily...as for the filter any HOB or sponge filter will work just fine. Remember for any intake you must cover it with a type of sponge so the dwarves do not get sucked in....one of the easiest ways to lose them i think