It'd be one at a time until they made the restrooms even too small for that.
Back in the 70s, Russian passenger planes were nothing but converted WWII cargo planes with seats. Not a single bolt or screw or rivet had been tightened since the war. The boarding tunnels hadn't found their way to Russia so you had to walk half a mile to the plane and just like tanks, you boarded up the back. Then you'd sit for an hour until someone's poor 80 year old grandma would drive up in a truck, climb on the wing and fuel it up. No gauges so she'd have to peek in to see if there was enough fuel (where's that rosary!). The seats were all broken so that you'd hope someone with long legs was behind you to hold your seat back up. They hadn't figured out to tack the carpet down the aisle so it was constantly rolled up. Snacks were always a Dixie cup of mineral water and a lemon drop. Barf bags didn't exist so passengers had to bring their own newspapers. I never had the courage to unbuckle and go back to the restroom so crossed my knees until we landed. I never saw anyone else brave enough either so took that as a sign. Well, the very last plane ride on the way back to Moscow to catch the flight home (you don't know what a welcome sight that blue circle on Pan Am was) I HAD to go so said a prayer and stumbled back to the restroom terrified of what I'd find. Ahhh, the heavens parted and angels sang! The restroom took up half the back of the plane with a lounge with a 1940-50s era couch and a couple of upholstered chairs and end tables. The toilet area was behind another door and was spacious. I spent the rest of the flight back there and still no one ever ventured in.
Back in the 70s, Russian passenger planes were nothing but converted WWII cargo planes with seats. Not a single bolt or screw or rivet had been tightened since the war.