From what I know, there are four world grand prix qualifying events: Europe, Asia, Japan, USA (each of which have 'feeder' preliminary events, held in cities like Amsterdam). The Japan GP might be the exception.
Four additional fighters who have performed well in GP events that year are chosen.
Then, the final eight men in the previous year's WGP Final are chosen to round out the final sixteen (called Final Elimination in (insert city name here)). The fighters only fight once that night, which leaves eight for the WGP Final.
K-1 Max is the middleweights (155 pound limit). Buakaw, Masato, Kraus, etc. They also have WGP events like the heavyweights.