Him/her, her, their... What to choose for referring to somebody whose gender is unknown?

My subject was not dear Max or any other dear A2Ks. By the way excuse me if it implied that. I had just seen he or she/he/she/they all of them in literature. I just wanted to find a guideline what to choose in such cases. Now I guess that in cases where I need to keep safe I must use he or she and in other cases I can use they without worry.

That was very helpful. I think I got my answer. Thank you. Thanks all.

Nah son, I get tied up in technicalities. You're doing fine