You have to initialize your data table
However, I would not recommend to use this approach. The reason for me to say that is, at the time of initiating this workflow, by deault the rows of the datatable will be set to 0 (unless you are passing a datatable through arguments). So in that case, at the initiation, your variable will also get assigned with 0. This value will not change automatically later even though you add data to the data table within the workflow. You will anyway have to use a assign activity to get the latest row count into the variable.
So the best way would be to use the assign activity to get it. it will be a more accurate way of doing it…