6 cells, nominal 3.7V each, so 11.1V/3.7= 3 in series
6 cells / 3 in series = 2 parallel runs of 3 in series
2 parallel to reach 5200mAh = 5200mAh/2 = 2600mAh per cell.
You do not need to get 2600mAh rated cells, could use a bit higher or lower, and remember that was only their original rating not the now old degraded capacity remaining so even if you bought a couple 2600mAh cells, they won't match the rest in true capacity.
You should replace all 6, same make/model, should use decent major brand capable of (ideally) over 10A drain rate.
However once you rebuild the pack, you will probably need to run a calibration routine in your laptop bios or windows app to get the right runtime estimates for the new pack.
That kind of math is mostly reserved to crappy chinese jumpstarters and power banks. Any pro battery pack from a reputable company should be rated like the laptop, or tool batteries, etc are, where it is the mAh capacity at the series voltage it outputs. Unfortunately the madness is spreading, more reputable companies are now considering rating in this unorthodox manner to try to look competitive with the faux generic product specs.
There are some generic brands making a compatible C4500bat-6, but unless you find someone who did a teardown, there is no assurance ahead of time about what cells are in it, even whether they are new or used, so buying major brand cells from a trusted source to rebuild yourself, is the surest way to end up with a better capacity and lifespan.
