2-3 more PCAs are probably needed. It seems like you are in a position where you need to look at a team based nursing model as opposed to a primary nursing model if you are truly not able to do anything about staffing levels. Start looking at Lean management to cut out tasks that do not add value (value does not just mean $$$) and to look at the actual set up of the unit.
Also, are the nurses stuck taking on roles of a lot of other disciplines (Social work, house keeping, laboratory, case management, etc)? If so, it may be a good time to establish boundaries and get non-clinical tasks off of their plate.
What do your nurses tell you they are struggling with? Why are they leaving?