The number determines the number of keyframes to be used in the loop, not the cycles. There's no specific expression for that. Simply split the layer.
what you mean for SPLIT the layer?
a video of 1 minute
is there a way to cut the video example to 20 secs and than paste the last 40 secs later? or i have to create 2 layer one where i put 20 secs and another where i put 40?
Set the CTI to the time you want to split the layer. Select the layer. Split the layer by pressing Ctrl/Cmnd + Shift + D or by going to Edit>Split Layer (Keyboard Shortcuts are on the right).
This will duplicate your set an out point on the existing layer and an in point on the new one.