Currently the limit on a single GEDCOM is 5000 people. If you want to upload more than that, you'll need to split your GEDCOM into pieces (eg. each of your grandparents' trees). This can be done on many desktop genealogy apps.
Keep in mind that the main reason for the limit is that it can be difficult for one person to manage so many profiles. Once you upload your GEDCOM, you are responsible for responding to merge requests and other collaboration on those profiles. Keep that in mind before uploading multiple files. It's best if you get comfortable managing a small number first.
I hope that helps!