|Shhhhh, don't tell SDOT!|
If you wish to calculate how many bottles you'll need for your border, multiply the total running inches and divide by 3 inches which seems to be the average width of a wine bottle. So, for 10 feet, which is 120 inches, divide 120 inches by 3 inches. That equals 40 wine bottles. You'll need more than you think you do.
|Notice the indented bottom-ups on some bottles.|
If the bottles are on a slope, you might try a pole stake (like bamboo or a hardwood dowel) to hold the bottle more firmly in place. Measure the stake twice as long as the bottle, then with a rubber mallet, pound just the stake into the hole until enough is sticking up to fit well up just touching the bottom of the bottle (or is it the top now? How 'bout calling it a bottom-up). You'll need to put the stick in the bottle and mark the spot where the stake goes to get the spacing right. If you use rebar, then I suggest you pad it with an electrical tape or a rubber tip of some sort, especially where it may be in contact with the neck of the bottle and around the end of the stake or the bottle could break. This technique may also server you well if you're using bottles for raised bed borders, holding back a lot of soil.
Not only are you re-purposing a common item while adding an attractive element to your garden, the wine bottle border will help warm the soil, encouraging happy plants. Depending on the size of your project, you probably can't drink enough wine to construct your border within your lifetime. If you can, well then.... So, I suggest you become a dumpster diving recycling bin raider to collect enough for your weekend project. Ask your wine drinking cohorts to start saving bottles for you. Just make sure you pick them up in a timely manner, before the beneficiary's garage gets overrun. Be forewarned however; upon hearing about this idea your friends may get inspired enough where they'll be competition with you for a somewhat limited resource. Things could get interesting, like an old Woody Allen movie interesting.