20 bit instructions. => $2^{20}$ different encodings or formulations of instructions possible.
Already explained the difference between an instruction and an instruction encoding here.
An address = 8 bits (because 256 word memory)
=> The "operand" field in the instruction format takes 8 bits.
there exist 8 two address instructions
Two address instructions == Two operand instructions.
So, $2^{16}$ different combinations of two operands possible, since an operand is of 8 bits.
These $2^{16}$ different operand combinations can be clubbed with 8 different types of instructions.
So, total encodings = $8 * 2^{16}$
1984 one address instructions
Similarly; $1984*2^8$ encodings possible for this one.
So, out of total possible encodings, 1AI and 2AI take $8 * 2^{16}+1984*2^8$ encodings.
= $2^{19}+507904=1032192$ encodings
Remaining encodings = $2^{20}-1032192=16384$