Hi Neiji,
Welcome to BuddyDev forums.
Thank you for using MediaPress.

To disable the multiple file selection, you can put the following line in your bp-custom.php

add_filter( 'mpp_upload_default_settings', 'mpp_allow_single_file_selection_only');

function mpp_allow_single_file_selection_only( $settings ) {
	
	$settings['multi_selection'] = false;
	
	return $settings;
}

That will limit the selection of one at a time but will not stop users from selecting multiple files. Now, since we don’t want users to upload more than one file at a time, we will take a little help from javascript.

Please put this js in your theme’s js file

jQuery( document ).ready( function () {
	
		
	if( _mppData == undefined ) {
		return ;
	}
	
	mpp.guploader.isRestricted = check_file_limit;
	mpp.activity_uploader.isRestricted = check_file_limit;

		
	function check_file_limit( uploader, file ) {
		//clear any error we had earlier
		jQuery('#message').remove();
	
		var size = uploader.files.length;
		//allow selecting/uploading
		if(  size <=1 ) {
			return false;//no restriction
		}
		//remove file from queue
		uploader.removeFile( file );
		uploader.refresh();
		//remove the feedback that we added
		this.removeFileFeedback( file );
		//notify error message
		mpp.notify( "Only one file can be uploaded at a time", 1 );

		return true; //yes, this should be restricted
			
	}

});

That’s all you will need to do. Please try it and let me know if that works for you or not?

Thank you
Brajesh