WooCommerce Product Add Ons Ultimate has an upload field which allows your users to upload files. By default, the upload field only allows one file to be uploaded but you can change that.
To enable multiple file uploads, select the ‘Allow multiple uploads’ field.
If you want to specify a maximum number of uploads, you’ll need to enable AJAX file uploads. This gives you the ability to allow multiple uploads and specify the maximum number of uploads.
To enable the AJAX uploader, go to WooCommerce > Settings > Product Add-Ons and select the ‘Enable AJAX uploader’ checkbox.
There’s a complete guide to uploading files here.