If your alphanumeric number goes beyond 8 or 9 digits, it will be difficult to remember, and also one small error may lead to other confusions.
Employee Coding System for Multiple Companies
For your 25 companies, assign letters from A to Z, rather A to Y. Next, sequence the employees as per their date of joining, like A00001, i.e., A plus 5 digits. First, list all employees in order of seniority, i.e., date of joining, and in the prior column, give 00001. This method should make it easier to keep track of employees with respect to their seniority in the concerned organization.