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?
Answer:
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.
Sources:
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.