How many staples to fasten 6/3 cable to studs?
You'll need to staple it down every 4.5', and within 12" of the boxes

This support requirement comes from NEC 334.30:


  334.30 Securing and Supporting. Nonmetallic-sheathed cable
  shall be supported and secured by staples; cable ties listed and
  identified for securement and support; or straps, hangers, or
  similar fittings designed and installed so as not to damage the
  cable, at intervals not exceeding 1.4 m (4&frac12; ft) and within
  300 mm (12 in.) of every cable entry into enclosures such as
  outlet boxes, junction boxes, cabinets, or fittings. Flat cables
  shall not be stapled on edge.