You do not need dozens of vehicles before basic fleet management becomes useful. Once several people and vehicles are involved, simple systems can save time and prevent gaps.
Complexity matters more than vehicle count
Five vehicles with different drivers, sites and uses can be harder to manage than a larger standardised fleet. Look at the administration, not just the number.
Start with a simple vehicle register
Record registration, driver or custodian, service dates, MOT, tax, finance, insurance and replacement date. A clear register often solves more problems than specialist software.
Set responsibility
Someone should know who checks documents, maintenance, defects and renewals. Problems arise when everyone assumes another person is dealing with them.
Add systems as the fleet grows
Telematics, fuel reporting and fleet software can be useful, but introduce them because they solve a real problem rather than because larger fleets use them.