There are all sorts of arguments, regularly aired on Pprune, as to the the merits of two vs four engines but, for commercial aircraft, it's purely a question of economics. If you can do the job with two engines rather than three or four then two will be better from an economics perspective. I don't know what the maximum thrust available for a twin installation is from a practical point of view but assume that there will be issues of fan size, ground clearance, gear length etc and I imagine that a 125k/lb thrust GE90 on a "777-400" is probably about it.