I dont want to join public sessions, me and my friend have been thinking of buying it but we wonder if its good to be on a duo or if we need more friends

None of the vanilla subs really fit for 2 people unless you want to add a reactor (and wiring) to a respawn shuttle, which is quite difficult. However if you go to the Steam Workshop and subscribe to a sub like "The Eel" or "Sweet Pea" that are meant for 1-3 people, it's a lot easier to handle than a large 4-8+ person sub.

My friend and I manage fine as a duo in the Dugong, though we usually add a couple bots just to do general repair work. All you really need is a pilot and a gunner most of the time.

Barotrauma is one of those games that has a very unique feel to it. It invokes a feeling of constant tension as you walk around the sub or look at the sonar, waiting for whatever lives in those depths to come and try to destroy your vessel. Along with traitors that have missions that handicap the effectiveness of a crew and with how many things can go wrong on the ship, you can expect pure chaos to come to the ship eventually.