There are 5 cases being reviewed. It doesn't meant that there have only been 5 cases.

In the article it states:

*Multiple sclerosis (MS) with onset in the setting of acute SARS-CoV-2 virus infection has been reported, and reactivation of MS following non-mRNA COVID-19 vaccination has been noted, but there have only been three reports of newly diagnosed MS following exposure to mRNA COVID-19 vaccine. The association cannot be determined to be causal, as latent central nervous system demyelinating disease may unmask itself in the setting of an infection or a systemic inflammatory response. We report a series of 5 cases of newly diagnosed MS following recent exposure to mRNA COVID-19 vaccines. Latency from vaccination to initial presentation varied. Neurological manifestations and clinical course appeared to be typical for MS including response to high dose steroids in 4 cases and additional need for plasmapheresis (excision and treatment of someone's blood or replacing their blood) in one case. *

https://www.sciencedirect.com/science/article/pii/S016557282100312X