Thanks15fps is plenty in most applications.
10fps works in area's without fast motion (foot traffic) once you get used to viewing it.
I have a couple of cams in non-busy areas that do fine with 5fps.
It also depends on the bit rate you choose.
Using VBR will save some space.
Study this> Cliff Notes
I used the calculator and with 12 tb for 15 4mp cameras, it only gives me like less than 2 weeks.