First my standard disclaimer, that I am electronics n00b, and just started fiddling a while back, so plz validate with gurus here.
I was in the market for a battery backup solution for my project, in fact, I still am - just that it got pushed down in the list of priorities. In my research I found that the power-management IC's came accross, which assume LiION as the battery-power source, put down strict limit on no. of cells that can be charged with it, at a time. Apparently, charging LiION batteries is a complex or non-trivial algorithmic exercise, where adjustments are required and caution needs to be exercised so as not the damage the battery or in worst case, case an overheating & explosion. I would presume the power-management IC then to be limited by it's computational resources (i.e. how many computations it can do, to keep check that charging process is in control) is limited. And the result is, you need multiple such IC's to charge multiple LiION cells. IIRC, there are clear application-notes for specific Power-Management IC's (I think I checked TI ones), where such details are mentioned.