It has been calculated, from the number of worms found in a measured space in a garden, that there must be 133,000 worms in one hectare of land, or 53,767 in an acre. There may be fewer worms in a field - maybe half as much.
Then again, it depends what you class as a worm. There are many types of worms and they can be found absolutely everywhere on earth - in fresh water, seawater, soil and the bodies of plants and animals. Each group of worms has a vast number of species - for example there are more than 90,000 species of Nematoda - tiny threadworms called nematodes, too small to see clearly without a microscope. Some are useful, eating bacteria or killing insect pests and so on, while others do a lot of harm to the roots of crops.
Therefore, there will be more than 100 million in an average acre of farmland.