No problem, the answers are as follows:
1. There is no limit as such, but this kind of thing is greatly effected by your host/server setup. If you're concerned about speed issues when your site becomes very busy, you may wish to read this
WordPress codex page for more information on how you can deal with a high traffic
WP site.
2. You can find working, live customer Vantage sites on the
AppThemes customer showcase.
3. AppThemes does not offer customization services, however we do have a
certified partners page where you can find developers for hire. If you make contact with them directly with an outline of your requirements they will be able to provide a quote for the work. Thanks.