I've thought hard about this before, and I eventually just accepted that it's pretty much impossible to answer this question accurately.
Zoophilia is still considered a heinous crime, often compared to pedophilia, making it clear that whoever's into it knows that they should never talk about it. Then, breaking down the kinds of people who are into zoophilia or are active zoophiles, we have groups such as:
• People who openly come out as zoo and just don't give a fuck, which is a teensy minority within a minority that often gets absolutely shunned by everyone else;
• People like us, who happily talk about it in a specific forum online or in closed groups;
• People from small towns, countryside, farmlands or regions with very little discussion on the matter, who casually have some level of intercourse with their pets or farm animals due to the lack of drama around it, and sometimes don't even know the word "zoophilia" let alone the existence of a whole community around it — even if they talk about it openly, they're too disconnected to make any notable impact in a study;
• People who silently watch zoo porn, never actually having sex with animals or getting in any kind of conversation on the topic with anyone;
• People who would do or do have sex with animals, but it's either only a very casual thing in their sex lives, or "that one thing that happened once out of horny curiosity".
If you think about it, save for the first one, none of these groups would want to talk about it. A big, anonymous study would have most people cringe and ignore it, and no one person or entity big enough on the internet would want to conduct such a study. And if they did, a lot of zoos would probably still lie.
Although, considering how many pets are out there, how much humans love them and how horny we can often get, though, I'd say it's... actually more common than we think!