I'm a big fan of buying wine in 3 bottle lots as much as possible, as it gives you so many more options for enjoyment and a lot of advantages over 1, 2, 6 or even 12 bottle purchases. Over the many years that I have collected wine and built up (and run down) my wine cellar I have the following approach to buying wine:
1 bottle - for speculative buys. When I want to try something completely new to give it a go and find out of you like the variety/style/region/producer. Single bottles work well in our wine club cases when you want to try as wide a range of wines as possible. Single bottle buys are also essential when I'm looking to get that one-off special occasion wine to celebrate a birth-year, or to give away as a gift. And when I'm in the store I'm always tempted by new arrivals so its easily the most common quantity in which I purchase wine.
2 bottles - I usually know the producer and like the wines but its at the top of my price range and I want to put them both away for a few years. Or as is more common in my household, its so I have one bottle to share with my wife, who is also in the wine business and we love to try everything together as much as possible, and another bottle either of us can enjoy out with friends on separate occasions. I'll also buy 2 bottles of what I call drinking wines (usually in the £12-£25/bottle range) for taking to BBQs or a friends place to share, or to have something in the fridge ready for a spontaneous glass when the mood strikes and I don't want to open something more serious. We drink a lot of white wines around these price points, for example, during the summer.
3 bottles - this is my ideal buy, the sweet spot, depending on what my budget can afford. With 3 bottles you have one bottle that you can try more or less immediately - for me that's within the first month of purchase but sometimes that's within 20 minutes of bringing it home! It also gives me a sighter as to when I think the other 2 bottles could be opened. Sometimes we'll open it and think its in the zone now and therefore aim to enjoy the remaining bottle over the next 3-12 months. Other times we'll both agree it's not ready and it needs time in the cellar. In this way we have another 1-2 bottles to either drink at much later dates, or to share out. It gives me lots of flexibility. The other reason I like 3 bottle purchases is that there is now a lot of wine that I want to buy! We live in a golden era for wine appreciation and there is now excellent wine being made in literally hundreds of regions around the world. We have plenty of favourites but also are constantly discovering new things. If you want to drink as widely as possible (it's all learning!), counting vintage changes too, then keeping yourself to 3 bottles helps you spread your drinking pleasure far and wide. There are some great bargains in the wine world and 3 bottles of something exceptional can sometimes be very affordable when bought with the cellar in mind. For me this is getting something at £30-70/bt (cheaper under bond) and laying it away. German Grosses Gewaches (Grand Cru) Rieslings are having a dream run and are great value. So too is Grand Cru Chablis from producers like Droin. Ridge Geyersville and Lytton Springs. Produttori del Barbaresco, Barbera from Piedmont, Cabernet Sauvignon from Meerlust, Beaulieu Vineyards or Wynn's, Loire Cabernet Franc, Penfolds Bin 389, Eben Sadie single vineyard wines, village or sometimes Premier Cru Burgundy from good producers, Cepparello from Tuscany, Terre Nere Crus, Chardonnay from Margaret River and California, Piedrasassi Syrah from California. The list of options goes on and on.
6 bottles - this is when I know exactly what I want and the budget allows. Sometimes its a case of something I know I will always enjoy and which can age for many years. We have a number of wines like this in our cellar. It could be a case of Barbaresco or Barolo or Burgundy en-primuer, or often something from the northern Rhône - these are probably our home favourites for the cellar. It also works well for wines that we know are superb value and we're happy to grab a 6-pack for seasonal drinking. It could be a lovely Bourgogne Passetoutgrains or Rouge, or Cru Beaujolais for quaffing over summer, or my favourite household go-to whites like Argyros Assyrtiko or Rafael Palacious Louro, or Donnhoff Hollenpfad Riesling. We love these wine at any time.
12 bottles or more - If you're catering for parties or events then its obviously the ideal way to keep make everyone happy on the right budget, and you always get the best prices for buying in quantity. But otherwise buying in 12 bottle quantities is rare purchase for our 2-person household, but occasionally we go deep on wines that we absolutely love or that we know will age for many years. I went long on 2002 Dom Perignon many years ago when it was released and I've never regretted it. Whenever we mark something memorable - family wedding, family reunion in Australia, significant birthday, major life success - we have a bottle ready to enjoy and it always brings a smile. 12 bottles are great for wines that you know and love and are happy to drink anytime, and at any age. This is the classic way of buying Bordeaux, for example, and its investment that repays itself many times over in pleasure and satisfaction. Its also a well-kept wine trade secret that buying non-vintage Champagne from a house you love, and ageing for another 2+ years amplifies the pleasure and experience of enjoying Champagne. Keep doing this every year and you'll drink fabulously unique Champagne for the (hopefully!) rest of your life.