Depends on the ethnic composition of your employees. Some cultures have only a small number of first or last names. 40% of Vietnamese people have the surname N

Depends on the ethnic composition of your employees. Some cultures have only a small number of first or last names. 40% of Vietnamese people have the surname Nguyễn, if you use the first initial + last name system then by the birthday paradox, there is a 50% chance of a first initial collision with 6.5 Nguyễns in your organisation. That means that if you have more than about 16 Vietnamese employees, you have a 50% chance of an email id collision. It's actually higher than that because Vietnamese-American first names are often English, so the distribution of first initials is not evenly distributed.