Does a sheath of grain contain one stem or many stems?
A sheath of grain contains many stems, not one. It is a congregation of many grain stems that have been brought together, forming a single bundle. This is how the term is used in the instructions about the first fruits offering.
Les explains that a sheath is not a single stem but a congregation of many stems of grain brought together. He is teaching from Leviticus 23, just after Israel came out of Egypt and gathered around Sinai to receive the Law. In verse 9, the Lord tells Moses to speak to the children of Israel: when they come into the land and reap its harvest, they are to bring a sheath of the first fruits of the harvest to the priest and wave the sheath before the Lord. The word is 'first fruits,' plural, because the sheath represents many stems collected as one. So the sheath itself is a bundle of many stems, not one.
Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest: