Hi, a few things here:
Are you uploading to a File column or Notes within the record?
If File column - this can only support a single file
If Notes - Power Pages support adding multiple files at once, you can enable this within the Form configuration.
With regards the requirement of removing old files, you might need some custom logic there, either a dataverse plug-in or something to remove the files before adding the new ones.
As an easier process, you can prevent the user on the Portal to add new files unless they remove other files first, you could achieve that via JS but it's more manual.