In my opinion less is the correct answer. However, the answer provided by the grammar book I`m studying is fewer. Is attendance countable or uncountable ?
Both the CALD and the OALD show
attendance both as uncountable and countable noun(as in the persons attending)
I've come across
fewer arena attendance
as well as
better attendance and fewer expulsions
One more question, please: is there any other word or expression for ballpark?
I'd use baseball stadium
hope this helps.