3 engineers. I (and a teammate) carried a pager for about 6 months but then Search SRE took over ops; it shared backends with other search systems and was primarily a frontend innovation so the marginal cost was minimal. There was some ongoing AdSales cost for it - basically we built a tool to visualize related queries for your search [1] and then AdSales got a hold of it and said "Woah, we can show this to clients and tell them 'These are all the other queries you should be advertising on'", and then the clients spent a whole lot more money advertising on a whole lot more queries.

[1] https://www.ducttapemarketing.com/using-google-wonder-wheel-...