The guide given by @Okiryu is good for bulk fill of large quantities. As noted in his reference, the information is from the U.S. Naval Facilities Engineering Command (NAVFAC) and has long been referenced as an authoritative Geotechnical Engineering source. The DM 7 series of manuals are excellent reference materials.
For individual structural fill applications, I tend to be a bit more conservation and use the general building code guideline in the U.S., which is 1 test per 2500 sf, per lift. In addition to that I also specify that without regard to the area, at least 3 in-place density tests are to be performed for any structural fill. Further, for strip footings, I specify 1 test per 50 linear feet, but again, no less than 3 tests.
My reasoning behind 3 tests is this:
1 test can be an anomaly…either good or bad
2 tests often confuse an issue…one is good, one is bad…which is correct for the larger area?
3 tests at least provide a referee test to help with the interpretation of two tests and the extrapolation of such.
I continue to apply this “3 test minimum” to almost every application of construction materials testing and have done so for over 40 years. I don’'t plan to change