Mathematically speaking, you are correct. The STV system does allow you to select your 1st, 2nd, 3rd, 4th, 5th, 6th favorite… right up through the end of the list. But as a matter of practical consequences, allowing unlimited choices will capture LESS of the community's true intentions, not more.
Think about it: In an election, you typically have a reasonably strong conviction about who you would like to see win. But if that person doesn't win, single transferable votes (STV) let's you pick a 2nd person you would vote for in an "instant runoff" of the remaining candidates. Failing that, you pick a 3rd preference. But by time you get to your 6th, 7th, 8th, 10th, 30th vote, how much conviction do you really have in expressing a preference?
So, if your top three candidates do not win, what happens? If the highest voted candidate has not reached their quota, it will apply your 4th choice, then your 5th choice, … your 15th choice, … your 23rd choice, in sequence… until some candidate reaches their quota and is declared the winner.
It's true that your vote will eventually only be counted once, but what did your vote count towards? Probably a candidate that you barely know or care about one way or another… selected only because you were compelled to select someone… anyone.
The end result is that your "just select someone, already" vote has likely off-set another voter which probably had much more conviction about who should win from that latter pool. More choice is not always better.