Both "Sopas de Letras" and "Sopa de Letras" translate to Word Search (literarly "word soup").
Because "Sopas de Letras" contains an 's' after 'sopa' it can be considered plural. So "Sopas de Letras" would be many Word Searches, while "Sopa de Letras" would be a single word search.
However, when I search on Amazon for "Sopas de letras", the majority of the word search books use "Sopa de Letras", even though they contain more than one word search. Similar results can be found on Google Books when using Spanish language and the same with Google Ngram when using Spanish.
Why is this?
Also, would it be incorrect or sound strange to title a book of Word Searches "Sopas De Letras" instead of "Sopa De Letras"? Same with "Sopas de números" and "Sopa de números": Would the use of "S" to make it plural sound strange?
I am a beginner Spanish learner, so would appreciate simplified explanations.