How long is the output of regression Lstm

I have seen some sequence to sequence regression examples using lstm layer ending to regression layer. Its output is set to sequence ( lstm layer) . What i dont understand is what factor sets the lenghth of output sequence.

I have a data set of various sequencies with various lenght. And their label is as long as their length.

What will be the output of the regression layer at training? A sequence how long??

How long will be at when testing a new sequence?

When i feed a sequence of 10 columns at training, i want to get 10 elements, when i feed 20 cols, i want to get 20 elements. As this is my ground truth data also. Also at testing i want the same. How do you set this?