How to model weak entity in ER diagram?
Room should have PK of (hotel_id,room_number).
Weak entities have compound keys with the strong entity key/foreign key as the leading column(s).
There are people who disagree with this approach, but they are wrong.

How can i find the room belonging to specific hotel which has the different number of beds limiting the attributes in the domain of my problem

Something like:
select *
from rooms
where hotel_id  = 1001
  and beds = 3