I'm sure there's a formula to figure out how much block you'll need but it varies from block to block. Check with your supplier on how to figure it out.
Use a crushed gravel base. Tamp the gravel down with a hand-tamper. You needn't have a gas-powered compactor based on the height of your wall. Just put in 3" of gravel and compact it with a hand-tamper. A 6" tamper should be fine. You can NEVER over-tamp with a hand-tamper. If you put in too much gravel at once, it won't tamp properly, and when water gets in there, and freezes, the gravel will 'move'.
Drainage is the key to a properly constructed wall. Again, since you're under 4' in height, you needn't tie the wall into the hill behind. If you're installing on a level surface, drainage is more difficult. You may need to dig out MORE dirt behind the wall (for gravel back-fill) than normal since you can't really divert water that well on level ground. (The more backfill you have, the better the drainage ...) BUT, if you're installing on a slope, you can collect the water and channel it away with solid piping or perforated pipe. You still need back-fill area behind the wall but only the minimum of 12" or so. This is because water won't be 'sitting' under, behind, or in front of the wall.
When you dig out behind the wall, make sure the area you dig out is wider at the top than it is at the bottom. You want the 'dirt wall' to gently slope away from your retaining wall. Here's a picture to look at. DIY Retaining Wall. It's a link off of Orco Inc.. Just poke around the Retaining Wall link and you'll find the DIY info, Product Info, Technical Info, and more on retaining walls.
And lastly, I strongly urge you to use Filter Fabric in your installation. AFTER you do all your digging and BEFORE you put in any gravel anywhere, lay in the Filter Fabric. Its purpose is to prevent dirt, itself, from 'polluting' your gravel. Little bits of dirt over time will make their way into the gravel, thus, polluting it and causing possible failure in the landscape behind the wall. This fabric will allow water through but NOT dirt.
Special TIP: You have 2 choices when you get to the top of the wall (in terms of how to finish the job.) 1) YOu can backfill all the way to the top with gravel, or 2) you can fill to within 4" of the top with gravel, and backfill with dirt to grow grass. How to decide??? If the landscape behind the wall slopes TOWARDS the wall, I suggest gravel all the way to the top. Go ahead and backfill to within an inch or 2 of the top, cover with Filter Fabric and finish the backfill with some of that decorative gravel the sell at Nurserys and the Home Centers. HOWEVER, if the landscape behind the wall is GENERALLy flat, backfill to within 4" of the top with gravel and lay some filter fabric on top. Then, backfill with 4" of dirt. Tamp it down, then plant grass. The fabric will prevent the dirt from 'polluting' the gravel, yet, allow for drainage.
If you go with Option 2, there's a little trick you can do to insure that water won't run over the wall should it rain real hard. Backfill with dirt about 4" BACK from the backside of the wall. You see, if water was to run towards the top of the wall for whatever reason, right before it gets to the wall, it encounters this gravel and falls BEHIND the wall. This last few inches of gravel should be 'applied' the same way you apply the gravel in Option 1. (Backfill to within 2" of the top and use decorative gravel. DON'T forget to use Filter Fabric wherever dirt comes in contact with the gravel!) Oh, the reason I say you should put fabric over the gravel 2" from the top is because, yes, you'll be using decorative gravel. BUT, also, over a lot of years, the gravel may get 'polluted' from things like grass clippings, blown dirt, and so on. By only having to remove the TOP 2" of decorative gravel for cleaning, the fabric prevents 'stuff' from polluting the gravel below. How nice! After you've cleaned out the top 2", you can lay in the decorative gravel again, and you're done!
Whew!!! I'm sure there are more 'things' I can offer but my brain is working overtime. If you have any specific question or can't quite understand what I'm saying, e-mail me directly. My best to ya and hope this helps.