the opmask registers are normally 16 bits wide, but can be up to 64 bits with the avx-512bw extension. how many of the bits are actually used, though, depends on the vector type of the instructions masked. for the 32-bit single float or double words, 16 bits are used to mask the 16 elements in a 512-bit register. for double float and quad words, at most 8 mask bits are used.