One of the best pieces of advice is to make sure that load your layouts into the designers gallery... in the past I have had a few designers ask me to join their team after they saw my pages with their kits in their gallery.
I typically waited for a call... and if I got my nerve up, I did drop them an email asking if they had a call coming and that I was interested. The calls are typically announced on their facebook, newsletters, right here in the forums...
I'm sure each designer is different... however, I don't think the quantity of galleries is as important at quality of your layouts in that gallery... if you get on the team, they have their requirements as to which/ or how many galleries they would like you to load to.