Really, really depends on the deck. If It's super aggressive like Buzzwole, you don't really need any, since the space can be better utilized. But something like Bulu should be playing 2 minimum. Zoroark is a special case since Trade and Puzzles grab stuff, and if you can dig for it against Garbotoxin before they're up and running, It's only a matter of keeping the tool off of it. But then a problem arises, which is the reason players stopped playing Field Blower. How many tools and stadiums do Garb decks play? In total, about 10(2 Parallel City, 3-4 Band/Belt, 4 Float Stone). How many Field Blowers are you playing? 2-3. And because you can't remove more than 1 tool from Garb at a time(because of obvious rule reasons that are irrelevant), you essentially get to turn off Garbotoxin twice. You completely rely on your opponent not having another tool ready to go between their hand and potential draw power. How is that at all different from the argument why Altar of the Sunne is bad? Yes, I get it, totally different mechanics, but it boils down to: If your opponent has a replacement stadium or Field Blower, you just threw away a card from your hand. It would just be much easier to Guzma and KO a Garb than to dig and dig for a Field Blower and hope for the best.
Now, on the other hand, not talking about Garbotoxin you theoretically wouldn't need more than 2 Field Blower because you would only be concerned with Choice Bands enabling KOs against you and Stadiums, presumably Parallel City. And unlike Garb, having multiple attackers in play with Choice Band attached is possible, and probable. So on paper, you probably will be able to get rid of all of your opponent's Choice Band in 2 Field Blower. Now, decks that play stadiums(including Parallel City) don't have much use for Field Blower outside of this scenario, and if you're taking KOs, you could leave your opponent at 0 Choice Band in just 1 Field Blower, and you would have counter stadiums to play. And if It's a stadium mirror, you don't really care, since it likely won't affect the board state very much. A lot of decks can easily get away with 0-1 Field Blower, hence why they're doing well, surviving that Parallel/Garb meta. But again, it varies deck to deck. I'm just explaining why people are dropping Field Blower.
In summary, it's not as reliable as other options, and decks devoting large amounts of space to Field Blower are sacrificing possible power and consistency just to gain a slight edge against an already losing matchup. It's an answer that vaguely applies to the question, but I do feel like It's the best answer, since it gives you a better understanding as an answer than just an answer with little expmanation/context.