Originally Posted by RussellinToronto
"a limited number of" is a prepositional phrase modifying the subject "fields": hence, "are" is correct.
It may help to think of it this way: if you reduce this sentence to its essential meaning, it becomes: "fields are limited" (not "number is limited").
Just out of curiosity, can you elaborate on this? I thought all prepositional phrases began with prepositions (reducing the sentence to "there [is/are] a limited number you can go into")?
*Edit* Didn't see browser's post until after, but it still doesn't seem definitive to me.