Driving alone, the logbook is a personal habit — you remember where you went and why. The moment several employees share the driving, that memory disappears. As the manager you don't know whether someone kept their logbook properly until the MTA asks — and by then it's too late to fix.
What changes with multiple cars
With a single logbook the question is «did I fill it in correctly». With a company you add «did everyone fill it in correctly, the same way, on time». The risk is no longer one person's: if one employee's records are incomplete, the MTA can tax their reimbursement — and you, as the employer, foot the bill. The more cars, the more of these individual weak points.
Three things that must be in place
First, every driver needs a signed personal-car agreement— not one shared one, but each their own. Second, everyone must record in the same format, or the data can't be compared or combined. Third, at year-end you need those records as a single summary for the INF 14 declaration, without hunting down scattered files.
From spreadsheet to system
A small team typically starts with Excel, and there's nothing wrong with that. The trouble begins when each employee sends their own file in their own version, someone forgets a month, and you only find out while assembling the annual return. A system where every employee works in one place and the manager sees the whole picture at once removes that manual gathering — and the gathering is exactly where company-scale errors are born.