I assume you’re talking rimfire here. For some reason, RF benchrest shooters often seem to gravitate to different flags than the centerfire benchrest shooters use. I have no idea why. I’ve shot CF BR for years and just started RF BR, so I use the same flags for both—Grahams— as mentioned in one of the above posts. They are popular and very good flags, though there are many other brands that I’m sure are good, too.
Some choices you’ll need to make:
1. Double-vane or single-vane? Double-vanes were popular years ago (I started with them) but now I rarely see them.
2. Do you want a propellor in the front or a plain ball as shown in the photo?
3. What tail material? The two most popular are surveyors tape and sail tails. If you want to start a fight among benchrest shooters, just say that one is better than the other and watch the sparks fly

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4. You’ll need poles. You can use a tripod but a pole with a stake that you push into the ground is liable to stay upright better in a strong wind. Just make sure that whatever pole you use has a tip that fits the hole in the flag’s bushing. I’ve used Bill Dittman’s poles for a very long time and recommend them. Full disclosure: I’ve shot with him for 18 years, but I’d recommend his poles even if I didn’t know him. If you’re interested, I can pm you his contact information.
5. How many flags? Answer: as many as your brain can process. Don’t be surprised when each is doing something different from the rest. I have trouble just keeping up with one! Joking aside, for serious RF benchrest at 50 yards, 3-4 will do it. One is not enough.
The semicircular unit in your photo is typically called a wind indicator. When people use them, they usually just use one, in conjunction with the closest flag. I’ve not used one because with all the flags I see (it’s OK to look at everyone else’s flags, too) it would be too much information for my feeble mind to process. Before you spend money on one, you might want to get accustomed to reading the regular flags.
Dave Rabin