ice screws mostly come in a mix of sizes ranging from 10cm, 16cm, 19cm and 22cm. [grin] In some ways the answer to your question is; "as many screws as you can afford". but to be more realistic here's my thoughts:
3 - long ~22cm
10 - medium - a mix of 16 and 19 cm.
2 - stubbies - 10 cm
you will always want to have at least 1 long screw on your harness to create a V-thread. the other 2 will be used for belays, wherever you can find a long screw placement. the mediums and shorts are for wherever is appropriate.
FWIW I know people who carry mostly 22cm screws, the idea being that you can always tie off a long screw. that's not my feeling, but YYMV depending on your experience.