Five pieces. In order by size:

1) Lipault small bag for those 7kg flights. On other flights, I'll pack it as a spare bag in case I need to check more stuff on my return.
2) Samsonite X-Space spinner, my "small" carryon where there's no (or a higher) weight limit.
3) Tumi Vapor 20" Int'l carryon.
4) Tumi Vapor medium case.
5) Ricardo Beverly Hills 28" case. This one's my oldest, but I rarely use it. Generally only use it where there's three of us traveling and we need a second checked bag between us.

Edit: when I bought 2, 3, and 4 last year, we gave away 14 pieces of luggage.....