Gosh, this is definitely all guesswork, and as you said, there's a lot of subjectivity here. Here's my guesses:
90% – if we're calling "ticklish" being ticklish at any level that gets any kind of ticklish reaction or reflex on at least one part of the body. I'd say more like 75% if we're talking about something like fairly predictably evoking laughter or promptly trying to stop or avoid the sensation.
25% – About 1/4 feels right. A lot carry memories of negative experiences from childhood, many find it too childish and don't want to be associated with it, and many simply can't tolerate it. But I don't think as much as half absolutely hate it, nor so little that it feels rare.
50% – That might sound high, but based on how the question was posed, I think most adults have tickled their romantic partner more than once, and in different contexts. I'm sure for many, many people it has occurred at least once. Probably just a foreplay experience and maybe wasn't even attributed to the tickling by them. Most cases were probably never or seldom repeated. Also, a lot of people get easily sexually aroused when touched in general, so it stands to reason that tickling would secondarily arouse them, especially if done by someone they are attracted to. "The right person" can arouse the majority of people in ways they otherwise would not be.
10% – I think it happens more often than people think, but also, arousal does not fully equate to enjoying the experience. It is possible for someone to hate being tickled and be sexually aroused by it fairly consistently. How they feel about the total experience may more often be negative or dissociated with sexual activity even though they do become somewhat sexually aroused. If talking about sexually aroused in a way where they enjoy the total experience on a fairly consistent basis, probably less than 1%.
1% or less. It's a very common flirting behavior, it's a gateway to initiating touch, and it's playful and facilitates bonding. It's probably not very common at all for it to be a regular occurrence in romantic relationships, but almost certainly happens to the overwhelming majoraty at least once in each relationship, and so especially across many relationships.