A flagship must accommodate the staff

How damaging is it for an admiral to command from a ship not built as a flagship, and how many major ships receive flagship facilities?

The central constraint is space for the admiral's staff. Roughly one in four major surface combatants historically had suitable accommodation, while modern carriers commonly provide it.

Flag command imposes physical requirements on a warship. An estimated one in four Royal Navy and U.S. Navy cruisers or capital ships historically had suitable admiral's accommodation, while modern carriers are generally prepared for the role. Housing the flag officer is the small part of the problem. A staff of roughly twenty to two hundred people also needs berths, offices, communications, and working space.