Points of possible interest: To be technical, since 'a lot' is singular and 'lots' is plural, the latter would imply more thanks than the former. However, I doubt many people would make this inference were you to use either one. Also, the production of the sounds in 'thanks a lot' is easier because of the 's' sound running smoothly into the 'æ' vowel sound while 'thanks lots' calls for an ...
Thanks a lot is the ordinary use in conversation, or bare thanks! Many thanks is more frequent in writing, particularly in formal writing. Thanks much is unusual. Here's a Google NGram. Keep in mind that the underlying corpus here is printed works, so the more formal phrase is significantly overrepresented.
She replied: Thanks a lot! "Thanks a lot" is an alternative to the more formal many thanks, thank you so much, and the American English (so I've learned) thanks a bunch and thanks a million. And yet, for some reason, the written form nearly always sounds sarcastic to my ears, even when it's meant sincerely.
american english - "Thanks a lot!" Sarcasm much? - English Language ...
Thanks a lot. So "thanks", by appearances a noun, is standing in for the exclamation "thank you", which is historically a short form of the complete clause "I thank you".
parts of speech - How to parse" Thanks a lot"? - English Language ...
I often see people write " Thanks very much " and think it is incorrect English grammatically. The following are fine with me: Thank you very much. (Thank: verb) Thanks a lot. (Thanks: noun) Many thanks. (thanks: noun) But "Thanks very much" is mixing "thanks" as noun with "very much" for verbs. What do you think? (I'm talking about strict English, understanding in casual conversation or ...