What would be your advice as an agent in regards to having novels authored by multiple writers? What would be the limit to the number of people involved? Would having more than two effect the chances of representation and publication?
Whether multiple writers can create one novel, and the number of writers involved, depends entirely on the writers. If you're a member of a five-person writing group and you all get along fantastically well and are on the same creative wavelength, then it could be fantastic. But from my experiences working with writers, they're not typically creatures who play well with others once they're in front of a keyboard. Also, there's the issue of consistency of narrative voice - it's very difficult to have a cohesive narrative voice if there are multiple writers - and if you're about to say 'But we'll all write in different voices', please don't ever send me that novel ... too many narrative voices just confuse most readers, including me. A novel is often a slow seduction - the reader is lured into the story, usually by the voice. If there are multiple suitors, we just give up.
On a practical note, it's rare, it seems, to see multiple authors listed on covers if the books aren't sci fi or fantasy. Unless all or some of the authors involved have a successful publishing history, it would be best to use a pseudonym - one name only - to represent all of you.