If you look at the datasheet, Register 0x1C has these bits
bit 7 = XA_ST
bit 6 = YA_ST
bit 5 = ZA_ST
bits 4:3 = AFS_SEL[1:0]
and you want to set AFS_SEL to the value 2 which is binary 10 which means set bit 4 of the register.
If you write that out as binary, it is 0b0001 0000 which is 0x10 (hex)
So, "writing the number 2" refers to the AFS_SEL bits which you then have to translate into the location within the register. Many libraries exist which define lots of handy constants and do all this translation for you.