Arrived a bit late here, but like other said before me, WTF is a colloquial slang, abbreviation for "What the f*ck?"

It is definitely used as a countable, many hits on a cursory Google search. Or take for instance this T-shirt here, once on sale at amazon.com:

Since it refers to a countable thing, you could easily say one more WTF or two WTFs less a minute.

A parallel example is OMG, BTW, which can used just the same way and probably most of these colloquial acronyms.

As to the point of "one WTF less" versus "one WTF fewer" there is quote some usage data in The Free Dictionary, to quote but one:

usage: Many usage guides say that fewer should be used before plural nouns specifying individuals or distinguishable units: fewer words; no fewer than 31 of the 50 states. Less, the guides maintain, should modify only singular mass nouns (less sugar; less money) and singular abstract nouns (less doubt; less power). ... Standard English practice does not consistently reflect these distinctions. The use of less or less than where usage guides recommend fewer (than) is common in most varieties of English: less than eight million people; no less than 31 of the 50 states; We did more work with less people. Though these uses are often criticized, they appear to be increasing in frequency.

So it is not even that it is colloquial or slang. Standard English practice is using less more loosely.

That is, you are correct in using both forms, WTF...

Arrived a bit late here, but like others said before me, WTF is a colloquial slang, abbreviation for "What the f*ck?"

It is definitely used as a countable, many hits on a cursory Google search. Or take for instance this T-shirt here, once on sale at amazon.com:

Since it refers to a countable thing, you could easily say one more WTF or two WTFs less a minute.

A parallel example is OMG, BTW, which can used just the same way and probably most of these colloquial acronyms.

As to the point of "one WTF less" versus "one WTF fewer" there is quote some usage data in The Free Dictionary, to quote but one:

usage: Many usage guides say that fewer should be used before plural nouns specifying individuals or distinguishable units: fewer words; no fewer than 31 of the 50 states. Less, the guides maintain, should modify only singular mass nouns (less sugar; less money) and singular abstract nouns (less doubt; less power). ... Standard English practice does not consistently reflect these distinctions. The use of less or less than where usage guides recommend fewer (than) is common in most varieties of English: less than eight million people; no less than 31 of the 50 states; We did more work with less people. Though these uses are often criticized, they appear to be increasing in frequency.

So it is not even that it is colloquial or slang. Standard English practice is using less more loosely.

That is, you are correct in using both forms, WTF...