Drop a few loads in the mouth(enough to fill it) at the beginning of the scene, have the girl hold it in there during the entire scene, even deepthroat with the cum in the mouth(if that is possible! lol!). If she spills it, make her take a few more loads.
Fuck em hard and and rough throughout the scene, give em squirting, leg shaking orgasms over and over again.
They should be allowed to swallow the cum only at the end of the scene once the remaining guys too have dropped their cum in her mouth/pussy/ass.
By the end she should have taken at least 20 cum shots!