The overall fee you pay depends on the fee rate (sat/B or sat/vB) and the actual size (bytes or virtual bytes).
Fee [sat] = fee rate [sat/size] * size

A single transactions with 1 input and 20 outputs is at roughly ~800 vByte.
20 Transactions with 1 input and 1 output each, are roughly 3-4 times larger in size. And therefore the fee would also be 3-4 times larger given you use the same fee rate.