How many batteries do I need to use lifepo4 battery 3.2V100Ah to make 12V100Ah

To form a 12V (12.8V) battery pack requires 12 (12.8V)/3.2=4, take 4 single cells in series;

100AH capacity requires 4 100AH single cells in parallel;

A total of 4*1 = 4 single cells are required.