To find the next number in a sequence you can try a few things to guess the next number
1. Check if it is an addition (n + 1),(n + 2),(n + 3)- If it is an addition the number will increase by the same for each step. For example 1,2,3,4,5
3. Check if it is a division (n - 1),(n - 2),(n -3) - If it is a division it will decrease for each step. For example 1,0,-1,-2,-3
2. Check if it is a multiplication (n*2),(n*2*2),(n*2*2*2) - If it is a multiplication or to the power it will usually increase by a large amount each step.

In this case each number is twice is large as the previous number.
8*2=16
16*2=32
and
128*2=256

256