Worst case, plant as compactly as possible in the largest area — 3/4 acre.

If you plant 24 inches apart with 3ft rows, you are using 6 sq ft per plant, covering 3/4 acre = 32670 sq ft, or 5445 plants.

If you plant on 48 inch centers with 4ft rows (16 sq ft/plant) in 1/2 acre, you only need 1361 plants.

Or, you can do the math this way:

R = feet between rows (between 3 and 4)
P = feet between plants (between 2 and 4) A = acreage (between 0.5 and 0.75)

#plants = 43560*A/(R*P)

Space rhubarb roots 24 to 48 inches apart in rows 3 to 4 feet apart for commercial growing. These distances can be decreased for plants in rows for smaller gardens (non commercial). Much smaller than this will seriously crowd the plants and result in a diminished crop and increase the likelihood of spreading disease. A 2- to 3-year-old plant, the Victoria variety, can grow to 4 feet (1.25 meter) in diameter and 3 feet (1 meter) tall. Plant the roots with the crown bud 2 inches below the surface of the soil. The hole for the crown should be dug extra large and manure, peat moss or organic compost should be mixed with the soil to be placed around the roots. Firm the soil around the roots but keep it loose over the buds. Water the crowns after planting. Give the plant 1/4 cup of 5-10-10 worked in to the top 10 inches of soil at planting time. Good garden drainage is essential in growing rhubarb. For home gardeners, planting in raised beds helps ensure against rotting of the crowns. Crowns will have a longevity of many years, but because of diseases and insects, it is normal to reset a bed after 4-5 years.