For the UBVRI filter system, the "standard" lists of standards are those devised by Arlo Landolt. The list of primary standards, mostly brighter than 10th magnitude, is published in Landolt, A. 1983, AJ, 88, 493. There is a second list of fainter standards with V magnitudes between 11.5 and 16.0, generally in fields close to the primary standards, in Landolt, A. 1992, AJ, 104, 340.
Peter Stetson has compiled a very large list of secondary standard stars in or near well-known objects, calibrated to the Landoldt standards. These standards extend to higher declinations than the Landoldt standards. The list is derscribed in Stetson, P. 2000, PASP, 112, 773.
Which, and how many, standards you need to observe depends on what you are trying to accomplish. You will be using the standard stars both to calibrate the wavelength response of your telescope/filter/detector system and to determine the variation of the atmospheric transparency with zenith angle. To do this successfully you need to choose standards that span the range of colors of your main targets, and to observe them over a range of airmasses (say, between 1.0 and 2.5). If you are trying to measure accurate colors, you should plan to observe at least a dozen standards. You should estimate exposure times quantitatively, but generally with the GOT you can expect 10- to 30-second exposures on 9th magnitude stars to get S/N>100 without saturating the CCD. The fainter stars will require longer exposures; consequently it may be best to use the brighter standards when possible.