I will be working at night in a office with men, where I will be the only girl. What are the halachos? How many men have to be present?
If the men are observant Jews, three men are required for seclusion with a woman at night.
If they are perutzim, meaning men who are not particular about modesty, and are therefore suspicious of licentious conduct, then even three don’t help.
See Rema, Even Ha-Ezer 22:5; Chassam Sofer, Even Ha-Ezer 2:76;Nitei Gavriel 20:2.
Note that in the presence of three men, yichud is permitted even if all three men are sleeping; see Nitei Gavriel 20:1; Dvar Halachah 9.