For that long, we wouldn't pack any differently than we would for one week. We wouldn't have enough clothes to not do laundry for 18 days or even 12 days. Number of suitcases is too hard to estimate since different sized suitcases changes it drastically. For my crew of 5, it would take two good-sized suitcases for my family of 5. (Good sized meaning right at the 50lb flight limit or a pound or two less). For a family of 7, I'm assuming some of your kids are older than mine, meaning bigger clothes. (My oldest is in a size 10/12 but my younger two are size 3 and 5 so they could count as one person). So, I'd add a third shared suitcase for your older kids. You could do one for you and DH, one for oldest two kids, and one for the three younger kids. Of course, it depends on your sizes and suitcase sizes. I'm short but DH is tall, so I think we balance each other out to two average height adults who are a healthy weight.