I don't think you need to look for a broker that has written loans for people that own 20+ properties, but one that has experience with multiple structures. Whether a person has 4 or 20 properties, if they are all in the one entity it's just a matter of dropping in the numbers on the serviceability calculator, knowing which lenders have issues with customers with large portfolios and if working in LMI territory, know the limits for each insurer. I know I've dumbed it down here so apologies to the brokers but this stuff should be basic for an experienced broker.
It's the clients with multiple properties in different entities, trusts, companies, where the rent is their primary income where the brokers have to know their stuff. This is the question you should be asking a broker. The number of properties that each of those clients has shouldn't really matter.
But the most important question to ask is "are you an investor"? If yes, ask how many properties they have. Without an investor mindset, a broker is a calculator who looks at the biggest upfront and trail (which when I was broking, Wide Bay were up there).
As a seasoned investor, I think it is up to us to have more input with the broker too. I only have 8 properties at present, but whenever I need to find more money from somewhere or am looking to buy another property, my broker and I have a strategy session to work out where to get the equity from, what vals we need to get, whether to top-up or refinance and looking for the best solution not just for the current need but to set up for the next purchase.