I want to book an hostel, we are two (two friends) so we need two beds in a room
I found this room:
What does the number of guests mean? how many should I select? we just need the hostel to sleep, not more
"Number of guests" means the number of people who will be staying in the room. A "guest" in this context is a customer of the hotel.
You are two guests; select "2" in the drop down. Usually his number will indicate how many beds in a dorm or other shared facility are reserved for you -- the remaining beds will be allocated to other guests. In the case of tents or private rooms, it also helps the hostel know how many you need. If you select 3 people for a 2-person tent, the hostel will reserve 2 tents.
For what it's worth, I recognize the interface in your screenshot. That particular website is rather confusing, and the price is per person, not per tent.