Here, from the National Multiple Sclerosis website, is a description of the four "courses" of MS:
If this link doesn't work, go to the website and look for a definition of the disease, and this information will appear.
Basically -- it's not the number of lesions, or even the location of lesions, that is a determining factor. It's how the disease specifically affects you, the patient. If you have clearly defined episodes, or exacerbations, or flares (all the same thing) where symptoms intensify, maybe new symptoms occur...and then a remission (although there may always be some residual symptoms), then you're in the most common category of "relapsing-remitting".
Generally over time, folks will start with relapsing-remitting, but then they'll realize that there isn't much remission from the last exacerbation. Then the doctors will decide it's "secondary-progressive". The treatment these days is pretty much the same: one of the interferon drugs, to slow the progression.
You can have lots and lots of exacerbations, but so long as there is a clear remission between them, you'll be classified as "relapsing-remitting".
How do doctors determine this? By watching the patient over time, seeing how well they recover from flares, see if they still have flares. For example, I'm labeled "secondary-progressive". I've not had any real exacerbation in years. Sometimes there is a small intensity of symptoms (like, increased spasticity, or loss of bladder control, or wonky vision, or...) and then it will fade in intensity...but it's always there. And I still have MS. Not worsening significantly, but if measured over time, worse now than when I was labeled secondary-progressive, 12 or so years ago.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....