Some quick autocomplete checks that I did:
- Google desktop autocomplete: 4 suggestions
- Google mobile / responsive autocomplete (iPhone): 5 suggestions
- Bing autocomplete: 8 suggestions
- Ask Jeeves (!) autocomplete: 8 suggestions
- Amazon autocomplete: 10 suggestions
- Play.com autocomplete: 10 suggestions
- Ebay.co.uk autocomplete: 11 suggestions
- Ticketmaster: 10 suggestions
As you can see from these quick comparisons that I did, results vary between the type of search that your user is performing - search engines like Google and Bing tend to return a small handful of results, like your teammate suggested, while e-commerce sites tend to return further results.
On occasions like these, I would sometimes choose to trust the logic of these large e-commerce-based companies, who will have no doubt A/B tested the hell out of their websites for optimal performance and results. From a UX perspective, personally I have no issue with seeing 10 results if I am trying to find a product on desktop; I would maybe narrow it down to 5 if your site is responsive. IMO, stick with your instinct and go with 10. ;)