First In your SSIS package you are going to save you row count into variable (for example vartotal).

Then when you open the script task editor, in the script section you see four items

1)Script Langauge

2)Entry Point

3)ReadOnlyVariables

4)ReadWriteVariables

In the readonlyvariables you need to select the variable vartotal

In the next step when you click on the Edit Script, you are going to use this varaiable with the code

string count = Dts.Variables["vartotal"].Value.ToString()