At what fleet size should I switch from a spreadsheet?

There's no fixed unit count -- the real trigger is whether more than one person needs to update availability at once, or whether you've had a double-booking or missing-unit incident a spreadsheet failed to prevent.

Can rental software track maintenance history per unit?

Yes -- purpose-built rental inventory systems typically log service intervals and repair history per individual unit, which a shared spreadsheet notes column doesn't scale to once your fleet grows past a handful of units.

Inventory Tracking for Rental Fleets: Spreadsheets vs. Software

When a rental fleet outgrows spreadsheet inventory tracking, and what a real inventory system needs to do that a shared spreadsheet structurally can't.