Give each tile four spacers for standard installations. You will need approximately 400 spacers for a 100 sq ft installation with 12x12-inch tiles.
Yes, if you remove them before the adhesive has fully set and clean them thoroughly. But they're inexpensive enough that most installers prefer to use new spacers on each job.
Yes. Natural stone requires even spacing to accommodate its natural variations and expansion and contraction.
Technically, yes, but not recommended. Spacerless installations are prone to lippage, cracking, and irregular grout lines
Remove spacers after a partial set of adhesive but before hardening completely—typically 20-30 minutes for quick-set products or 24 hours for standard adhesives.