Hi! A few things to clear up, bear with me!
(1) Of the four sites you posted, I can't overstate that only Tecendil should be trusted for transcription. The first two misuse tengwar fonts completely and produce gibberish, and Jens Hansen uses a mode with serious faults. It also confuses transcription (Latin alphabet > Elvish writing system) with translation (English > Elvish languages), which signals misunderstanding of the basics. The tengwar ("letters") are a writing system, while Quenya ("Speech") is a language.
(2) Next, and more importantly, we cannot say with absolute certainty how the Elves would have said or written "twenty-four" for a couple of reasons.
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In a late essay, Tolkien changed the word for "ten" from kainen to quëan; the largest number form attested from that conceptual period is neterquë "nineteen." Some Neo-Quenya authors look to an early draft word yukainen "twenty" and update it to match this new "ten"-word, producing *yuquain (with -quain as quëan's combining form), but we don't have confirmation that this is the form Tolkien would have used.
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Although the Elves ordinarily used a decimal (base-10) system, they also "preferred to reckon in sixes and twelves as far as possible" (LotR/1107) and had a separate duodecimal (base-12) system that "they used in all more elaborate reckonings" (VT47/14). There would have almost certainly been a special word for "24" in this system, its being the equivalent of decimal "20." In draft materials, it's yurasta, but we don't know what Tolkien conceptualized in the latest period.
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Everything we know about the numerical tengwar can fit a half-page, and some of the attested examples inter-contradict, which leaves number writing a very shaky endeavor in general.
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We are told that "for lists, series and the like, the tengwar from 1 to 24 (t, p, . . . , w) were used, just as we use (a), (b), . . . A sign such as a dot or bar above could be used, e.g. nt or ti" ("The Writing Systems of Middle-earth," Quettar #1). In this system, vilya would represent 24 and could potentially be written w or wi (or over-barred, but that's outside the formatting of this comment).
TLDR, this is a risky tattoo. We can't say for certain what Tolkien would have done, and whatever solution you land on could be invalidated by the publication of new material. If you're okay with that chance, here are my recommendations, in order of surety:
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24
This is "24" written numerically in the duodecimal reckoning. An optional dot or small circle can be written under the character on the left.
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w or wi
This is the twenty-fourth tengwa of the usual order, optionally marked with a dot or bar, as mentioned above. (The name of this letter means "air" or "sky," if that adds any special significance for you.)
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canta yuquain
These are the Quenya words canta yuquain "four (and) twenty" written out in the classical mode. I recommend this option least for the reasons outlined above.
Please don't hesitate to let me know if you have any questions. I want to make sure you're a very, very informed tengwar tattooer!
