I'd say you should probably get 5, but i'd plan on replacement after 3-4 so you can maintain continued Applecare support and sell it off to someone else at that point.
This is an important point. Don't forget to sell your old Mac. The initial buy-in costs you more with Apple stuff (but it's well worth it for the quality), however you can easily recoup around 50% a few years down the line when you upgrade. Your MacBook won't be worthless in two years like a PC laptop. But you do have to actually remember to sell it and not just throw it in a closet or something.
My first Mac was a 2005 iMac—the last of the PowerPC line—purchased in early January 2006. I didn't know about MacRumors or the WWDC refresh cycle, nor was I aware that anyone who was even paying a little bit of attention knew that an announcement of a shift to Intel processors was imminent for Apple's desktop machines. So I bought a brand new dinosaur right before the extinction.
To my surprise, that machine ran great for years. The Leopard upgrade actually sped it up. (Never expected that to happen after decades of Microsoft triggering hardware upgrades every time Windows got a refresh.) I had maxed out third party RAM when I bought it, and I think I swapped the HDD at some point, but otherwise it was the same machine, obsolete from the get go, and I used it for almost five years. Never had a PC that long. Just after the warranty expired, the main board crapped out on me. The local Apple Store went ahead and replaced it gratis—because it had been a known issue with that model—even though they didn't have to.
Anyway, when I traded up to a 2009 Intel iMac, somebody paid me over $700 for that used PPC iMac on eBay. I couldn't believe how well these Macs hold value. If I'd dropped $1,500 on a PC, there's no way I could've gotten $700 back out of it even two years down the road, let alone 4½.
And I'm still using that mid-2009 iMac, BTW. Again, maxed the RAM, replaced the HDD, and rolled my own Fusion Drive by swapping out the SuperDrive for an SSD. I initially wanted to wait for quad-core iMacs, and by the time Apple finally released those, it seemed like a Retina iMac would be just around the corner, so I kept waiting. In the meantime, I bought a 2011 MBA (still going strong) and a used 2010 Mini to be a home server and HTPC. My main machine now is a 2015 MBP, so, although I'd like to refresh the old 2009 iMac, it's hard to justify, since it's still running just fine going on 7 years down the line!