You have to try on bras to see if they work out well for you.
I do not own a sports bra, but many seem to be labeled with sizes like s, m, l, xl, etc.
I would suggest that if you do not know what size to get, go get fitted. If that is not practical for you, order the largest two sizes available for the style you want, and remember that a women’s L would generally be smaller than a man’s L, for example. Then work your way down. I saw some Bali bras recently that were sized that way, as compared to a 34 C or a 36 B, for example. Obviously your chest size directly impacts your bra size.
Good luck!
Chrissy