I bought a pack of abranet from PSI last year ...
I received 2 each of 120, 180, 220, 320, 400 grit pieces, approx 2.5 x 5 inches.
I took one "sheet" of each and cut it in half, and those half pieces lasted me for around 8 projects ... (but my projects include soapstone, alabaster, and a bit of metalworking here and there). One half-sheet set should go for at least 12 - 15 pens.
With abranet, I tend to sand a bit faster than with normal sandpaper, as it doesn't load nearly as bad or heat up nearly so much. Once the mesh loads up with something that doesn't want to dislodge, I try using it for wet-sanding (which works wonderfully, by the way!) on acrylic, to see if it'll clean up.
Once I reach the stage where the mesh starts to polish rather than sand on most of the set, I relegate that set to *non-critical* work and break out a new set. I still haven't used the last of that first pack, already bought another pack from PSI. If the set is too badly worn, I discard the worn out mesh (120 ... then 180 ... then 220) and keep the last few in a bin for hand finishing work. I wish they included even higher grits in the multipack, this stuff is just awesome! I'ld love to progress straight from abranet to micromesh, but for that I'ld want at least a 1000 grit and a 2000 grit mesh.
I always ensure that the majority of my sanding is done at the lowest grit possible ... 120 or 180 or 220. All the leveling and profiling is done at this grit, to promote the best life for the rest of them. The purpose of the remainder is simply to sand out the marks left by the previous grit.
I am not afraid to fold this stuff over to provide extra stiffness or padding, or pull on opposing sides to give a good straight profile or closely follow a curve. It is strong and very durable. Someday soon, I plan to pull the trigger and order a few rolls of it, so that I don't have to order it in little packs anymore. It is also sold direct by Mirka (makers of Abranet) in large rolls. I did note that they DO have higher grits available, and those are definitely in my plans.
All that having been said ... A single multi-pack of abranet from PSI should last you 50+ pens.