12 of these steel rollers per drawer for long drawers
http://www.mcmaster.com/#drawer-rollers/=jftm3g
6 on the bottom of the cabinet at the edges of the drawers
2 on the right and left side of the cabinet near the front opening
2 on the right and left side of the drawer near the rear to keep the drawer straight (careful to offset these so they don't hit the two mounted to the side of the cabinet for when you need to remove the drawer)
2 along the top of a brace mounted at the back of the drawer so that when the drawer is open, you're not scraping the back wall against the top of the cabinet...
Example of a cabinet I built for very heavy equipment using the steel rollers from McMaster Carr. The drawers are almost 6 feet long.