The 'higher end' motherboards will almost always come with 2 or more SATA cables. A lot of the cheaper motherboards at this point do as well, but it's less of a sure thing. If you buy harddrives "OEM" they won't have cables with them. A retail package will nearly always come with an SATA cable and mounting screws (and some CD that you will probably never need or use.)
The number of SATA cables you 'need' is pretty easy... one for every harddrive you buy and one for every optical drive you buy (like a BluRay drive or a DVD player/burner.) As cjl pointed out, most drives will come with more cables so long as you aren't buying drives from NewEgg, etc. that are listed as 'OEM.'