After buying a new digital camera, you can spend thousands of dollars on new lenses and other optional accessories, but the one add-on you can't start shooting photos without is a memory card. (Remember the days when digital cameras used to come with low-capacity memory cards that could only hold a few photos at most?) SD and Compact Flash cards are really cheap, and most people don't need the Class 10 cards rated for 90+ MB/s burst write speeds. You can get a 16GB SanDisk card rated for 45MB/s--The Wirecutter's current favorite--for under $20 on Amazon. But how many of these cards do you need? Is it better to buy a 16GB card now and get another card later or just buy a 32GB card to start? As I've come to realize, having more memory cards can actually become a hassle; there's an upper practical limit to how many memory cards it makes sense to own for each photographer's workflow.
I bought a 16GB card two years ago for my NEX-C3, back when pricing for SD cards was still hovering above $1 per GB. With the NEX-C3's 16MP photos, and when only saving JPEGs (~4-7MB), I could fit around 2000 images on a 16GB card, more than I'd ever need for a single day of shooting. That single SD card was enough for the longest time, and my photo processing workflow consisted of transferring those photos to an iPad using the SD card adapter and letting PhotoStream push them to my desktop. I also wasn't taking nearly as many photos back then, so I never regularly deleted photos from the card or flushed it clean until I reached its full capacity--which took six months. In my mind, the photos on the card was another form of backup, a way to access those photos quickly in-camera and using the card as a portable repository of my photos when I needed to access them while on a trip.
This strategy didn't work well when I started shooting and saving in RAW.
With the purchase of the Canon 6D, I bought a 32GB SanDisk card. RAW files from the 6D average 26MB each, so the 16GB card would only hold about 600 photos. That wasn't enough for heavy photography trips like WonderCon and Maker Faire, or if I planned to record any HD video on a day shoot. There's something nice about seeing the "999" photo estimate on a camera's top plate LCD to reassure me that I don't have to be stingy about how many photos I can take. When I brought two DSLRs to Comic-Con, I bought an additional 32GB card for Adam's 5D MKIII. In total, I had two 16GB cards and two 32GB cards. Plenty of storage for all situations. But maybe too many cards to juggle.
Having four memory cards for one camera gave me false sense of security about how much free storage I had available for any given outing. As with the NEX cameras I was shooting with, I kept on using the cards as pseudo backups, never making an effort to properly transfer all the photos to an external drive once they reached capacity. Lightroom was making a copy of each photos with every import, but those photos were just on one desktop drive, not backed up to an external system. This lead me to switch between cards before they were filled up, as I couldn't always remember which card had the photos I manually transferred over and which had photos still fresh from a recent trip.
300 Adam workshop photos on one card, 800 Dragon*Con photos on another, my cousin's wedding photos spread somewhere between the two. Crap, I don't know which cards I've backed up, and New York Maker Faire is tomorrow. Oops.
To avoid memory mismanagement today, I cut my memory card loadout to just one 32GB card. My personal policy is now to only keep photos on that card through the next big trip or event. I'll keep photos from short day shoots or product briefings saved on the card after importing into Lightroom, but after a trip to New York or taking backstage photos at Jamie and Adam's Behind the Myths show, I wipe the card after importing photos. That discipline forces me to not think of these cards as backup devices, and now I run an actual backup of my Lightroom library to an external 2TB drive. And every time I embark on a big trip, I get to see that  marker on the camera's LCD--the go-ahead to snap away without worry.
As for a backup SD card, I keep the second 32GB card in my backpack in case of emergencies or when we need storage for a separate camera like the GoPro. But both 16GB cards have been retired and now stay at home.
How many memory cards do you regularly use? What's your own strategy for data management on those cards?