This question was originally answered on 11/7/14. Please see below for an updated response to your question with new information.
Interestingly, there are many more than 1,000 separate seed companies that supply the many types of commercial seeds globally, and hundreds of seed companies that sell corn and soybean seed to American farmers. In fact, aggregate commercial sales by all seed companies account for about 40 percent of the total volume of seeds used globally. Of those commercial seed sales, two-thirds of the seed volume comes from private breeding programs and one-third comes from national or public institutions. The remaining non-commercial seed volume is seed saved and replanted by farmers. Monsanto participates in only a few crops, with the two largest being corn and soybeans. While Monsanto is one of the largest commercial seed companies, even in those crops the company probably accounts for less than one-third of global commercial volume. Therefore, if a farmer wants to plant seed from someone other than Monsanto, it’s pretty easy to do that.