How many parties can form a coalition?

Looking at the exit polls you could have a situation where Labour form a government with the SNP.
Lab/SNP 317 Con/Dem 306.
But if the tories add the DUP it could bring the result level.
So what then?
Labour call on Plaid Cymru to add a couple, but the tories counteract with UKip giving it a 1 seat lead.

It could turn into a situation like picking football teams at the local park, with George Galloway as the fat speccy kid who no one wants.