7*4 = 28 cells
each (say) 1.2 V = 36V
you would need: 28 voltage dividers - each taking current from the batteries.
You could use analog multiplexers - but cheaper and simpler to use 3 or 4 nanos.
I'd suggest you rethink your requirements - do you REALLY NEED to measure each cell? After all if they are in series you can not use or charge them independently.