I pulled a satellite image from Google maps of a section of Arlington National Cemetery:

By using the scale provided by Google and ImageJ, I determined that the average length between rows of graves is 9.7 feet, and the average length between headstones is 5.9 feet.

I then cropped the image to an area (~35,200 sqft) with no visual obstructions (trees, scale bar, etc.) in order to effectively count the headstones.

I performed 2 counts in this area. The first, I restricted the area of particle counted to the range 0.5-2.0 sqft. This was to estimate the number of headstone, and therefor the number of total graves in this area. The second count, I restricted the area of particles counted to larger than 3.0 sqft. This was to count the large light patches that in the image reflect recently moved earth, thereby giving an estimate of new graves in the area.

Total graves: 704

New graves: 3

Extrapolating this data out, it would estimate that there is 1 grave for every 50 sqft, and 1 new grave every 12,000 sqft.

According to Wikipedia, Arlington National Cemetery is 624 acres, or 27,900,000 sqft). So, if you further project my estimations (and assume that the same density is present across the entire cemetery), then there are 54,000 grave sites, 2300 of which are newly established.