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 only about 40 percent of the total volume of seeds used globally in 2014 (not sure of the last seasons). Of those commercial seed sales, roughly 2/3 of the seed volume comes from private breeding programs and significantly 1/3 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 their output 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 very easy to do that. People use Monsanto products because of the genetics/profitability not out of intimidation or fear.