"在册职工人数" can be translated into English as "the number of registered employees" or "the headcount of staff on the rolls".
"The number of registered employees" is a straightforward and commonly used expression, where "registered" indicates that the employees are officially recorded or listed in the company's records.
"The headcount of staff on the rolls" is a more formal or administrative-sounding phrase, with "headcount" referring to the total number of people employed and "on the rolls" indicating that they are officially part of the company's workforce.
Both translations accurately convey the meaning of "在册职工人数".