"other factors" can be treated as a singular group noun if you want, so I think both are correct. By example: "There are likely to be cows over that hill" - c

"other factors" can be treated as a singular group noun if you want, so I think both are correct. By example: "There are likely to be cows over that hill" - correct, many cows "There is likely to be a herd of cows over that hill" - correct, one herd "There are likely to be other factors" - correct, many factors "There is likely to be a number of other factors" - correct, one collection of many factors "There are likely to be a number of other factors" - correct, emphasizes the factors over the collection "There are likely to be other factors" - correct if you want to treat 'other factors' as a singular group. Up for debate. Also, "there're" is very hard to say, so using "there's" as a contraction for "there are" is, in my book, okay, even if it's not technically correct.