Is WTF singular? uncountable?
Solution 1:
I would argue that WTF is singular and countable, as it's s noun describing a part of speech, like a sentence or a greeting. It would be perfectly correct to say:
In his rant, there were four sh*ts and seven WTFs.
Therefore, "WTF" can be taken as countable:
Dylan's rant had three fewer WTFs than Amelia's.
Solution 2:
WTF is a countable noun and its plural is written WTFs. The adjective 'fewer' is better than the adverb 'less' - but the misuse of 'less' has become so common these days that it is almost acceptable in informal writing or speech.