If the release allows for more than 1 submission per producer, you can submit multiple tracks. The exact number can vary from 1-10. It is decided by the creator of the release. To find out for a specific release, simply go to the release page and in the maximum number of tracks per producer is mentioned in the requirements section.
If you’ve already submitted some tracks to this release and you're logged in, above the Submit button you will see a counter which will tell you how many tracks you could still submit to this specific release.

If you want to submit multiple tracks, you can submit them all at the same time or at separate times during the Open for submissions phase. Here's an example: In a release with the limit of 7 tracks per producer, you could for example submit 3 tracks today, 2 in a week, and 1 in two weeks.