SSN is an abbreviation for "Smoothed Sunspot Number".
How the sunspot number should be smoothed depends entirely on what you are doing. Every useful ionospheric model specifies the manner in which recent solar activity is to be conveyed. If wish to run HF propagation predictions using the VOACAP forecasting engine, for example, then you would generate a smoothed sunspot number in the manner it prescribes:
Additional information you may find helpful is here: