Subfloor sheet count is straightforward. What matters more is thickness, orientation and whether it was glued — because a subfloor is the one layer you cannot go back and fix without pulling up the finished floor.
Thickness follows joist spacing
Three-quarter inch tongue-and-groove is standard for joists at 16 inches on centre. Joists at 19.2 or 24 inches need thicker sheathing or an engineered panel rated for the span.
Under tile, three-quarter inch subfloor plus a cement board or uncoupling membrane is the normal build-up. Tile over a thin or bouncy subfloor cracks at the grout lines.
Perpendicular, staggered, and gapped
Sheets run across the joists, not parallel with them, so each sheet is supported by several joists. End joints stagger by at least one bay so no seam runs the length of the room.
Leave an eighth-inch gap at the panel ends. Wood panels expand, and a subfloor with no gap buckles at the seams.
Glue is what stops squeaks
Run a bead of subfloor adhesive on every joist before the sheet goes down, then screw it. Screws alone allow the tiny movement between sheet and joist that becomes a squeak.
Use screws, not nails. Nails back out over time and are the reason old floors develop squeaks that were not there when built.
OSB or plywood
Both are approved for subfloor and both are used constantly. Plywood is more resistant to swelling if it gets wet and dries flatter afterwards; OSB is cheaper and comes in larger sheets.
The practical difference shows up during construction, when the deck is exposed to rain. OSB swells at the edges and the swelling does not fully go back down, which telegraphs as ridges under vinyl and thin flooring.
If the subfloor will be exposed to weather before the roof goes on, plywood is worth the difference. If it will stay dry, either is fine.
Getting it flat enough for the finished floor
Every flooring product specifies a flatness tolerance — commonly something like 3/16 inch over ten feet. Most subfloors as built do not meet it.
Check with a long straightedge before ordering the finished floor. High spots get sanded or planed down; low spots get filled with a levelling compound, which is sold by the bag and is its own quantity to work out.
Laying a rigid floor over an out-of-flat subfloor produces bounce, gaps at the joints, and in the case of tile, cracked grout lines. The remedy after the fact is to take the floor back up.