"在册职工人数" can be translated into English as "the number of registered employees" or "the number of staff on the rolls".
"Registered employees" emphasizes that the employees are officially recorded or listed in the company's register.
"Staff on the rolls" is a more formal or traditional expression, where "rolls" refer to official lists or records, indicating that the staff are officially part of the organization.
Both expressions are commonly used to convey the idea of the total count of employees who are officially recognized or employed by a company or organization.