OK, I can swim at a pace of 10 to 18m / min. Usually 15m / min
After doing experiments, 3m/min OFF swim pace for every stage added.
Certainly need practice / and nose clipping / leashes / help @ surface beyond 3 stages to get proficient.
I found beyond 3 stages, till I practiced, got no further due to increased gas usage.
I've been told by those a lot more experienced, that 3 stages is what most divers with some training can manage efficiently. The DIR guys seems to have some pretty good drills / routines for managing lots of stages...
Beyond this, serious consideration of staging (cave or on deco trapeze/bottom of shot and lining), team bailout, drop tanks, support divers, surface support, team bailout, team bailout in bigger dive teams, team bailout carry different/complimentary deco gasses (e.g. you take 50% I'll take 80%), using scooters instead of swimming etc is advised.
Using some of these techniques (and a medium sized dive team), got up to 18 stages without too much hassle, apart from carrying them all to the water! This included set up, tear down and dive in one day. With larger teams, many days, as many as you like...
Carried 5 to 7 stages via scooter, and friends have done this swimming. Requires quite a lot of practice / kit fettling to get the stage rigging set up just right... Not necessarily different, but adjusted for you/cylinder gasses etc to sit/lie properly.
What experience with the dual rebreather and a little experience with sidemount rb's has taught me, is that 3 stages is the break even point i.e. beyond this Id say use a bailout BOB, its more practical. One/Two/Three stages, learn to deal with them/practice to get slick, it more practical with stages. Also depends on budget/frequency of use etc.
If solo, consideration of margins, splitting gasses between stages to create redundancy is a consideration, perhaps as well as viability of team bailout in larger groups/poor vis / not fixed lines etc.
Beyond 2 stages / large stages, steels are not practical in terms of available buoyancy...
YMMV, just my rough rules of thumb / limited experience. People highly skilled / practiced perhaps can handle more stages routinely.
The one thing I learned the hard way was NOT to compromise on volume of and appropriate bailout. Its expensive, frustrating, harder work, but critical.
Once doing dive that require many stages, they're usually deep / long bottom times and/or a combo of both.
Personally, when I started looking @ numbers for +5 mins/+5m, the gas volume needed for deco increased very significantly. I also think 1/3rds etc MAY not be enough to account for increased breathing rate, long swims, delay/confusion etc, so I wanted to use a BOB / staged rebreather to give a LOT of gas/time margin. One issue Im still wrestling with is how to safely stage a rb, and have the confidence its working if I come back to it.
May not be economically feasible, but food for thought, no bad thing to be generous with the bailout...
Im honesty I've done very few of those dives, so would be interested to hear from those more experienced.