It doesn’t matter how many bras you have in your closet, if you are working out - you need a sports bra to support you. Excessive movement can lead to discomfort, so the right sports bra is essential for your training. You will need to look for a sports bra that delivers the right amount of coverage and that is not too restrictive.
The right bra should fit snugly, but just like with the collar on a pet dog, you should be able to comfortably get two fingers under the band. If the back of the bra is riding up or you’re pulling up the shoulder straps throughout the day, then the band size is too big. Remember you can always adjust the hooks and the straps.
If your bra cup is moulded, your should fill the bra completely - you shouldn’t have extra room in the cup. Nor do you want to be spilling out of your cup, if this is the case you’ll need to go up a size.
Flap your arms about and wave them around. If the band goes up your back then it’s too big. If this happens, try adjusting the shoulder straps, or make the band smaller. If it’s still leaping about then go down a size.