In Genesis 46, Becher is listed as the son of Benjamin when Jacob comes to live in Egypt with the rest of the family at the behest of Joseph, now the second-highest person in Egypt.
In Numbers 26, Becher is listed as one of the family heads just before the nation of Israel enters the Promised Land.
What is the timeframe? We are looking at more the 400 years difference. So the Becher in Numbers could be a different individual than the one listed in Genesis.
Another theory is that Becher could have married into the Ephraimite family. McClintock and Strong Biblical Cyclopedia discusses this in the article under the subject of Becher:
As regards the posterity of Becher, we find nevertheless the singular fact of there being no family named after him at the numbering of the Israelites in the plains of Moab, as related in Numbers 26. But the no less singular circumstance of there being a Becher, and a family of Bachrites, among the sons of Ephraim (ver. 35) has been thought to suggest an explanation. The slaughter of the sons of Ephraim by the men of Gath, who came to steal their cattle out of the land of Goshen, in that border affray related in 1Ch 7:21, had sadly thinned the house of Ephraim of its males. The daughters of Ephraim must therefore have sought husbands in other tribes, and in many cases must have been heiresses. It is therefore possible that Becher, or his heir and head of his house, married an Ephraimitish heiress, a daughter of Shuthelah (1Ch 7:20-21), and that his house was thus reckoned in the tribe of Ephraim, just as Jair, the son of Segub, was reckoned in the tribe of Manasseh (1Ch 2:22; Nu 32:40-41). The time when Becher first appears among the Ephraimites, viz., just before the entering into the promised land, when the people were numbered by genealogies for the express purpose of dividing the inheritance equitably among the tribes, is evidently highly favorable to this view. (See Nu 26:52-56; Nu 27.) The junior branches of Becher's family would of course continue in the tribe of Benjamin. Their names, as given in 1Ch 7:8, were Zemira, Joash, Eliezer, Elioenai, Omri, Jerimoth, and Abiah; other branches possessed the fields around Anathoth and Alameth (called Alemeth 6:60, and Almon Jos 21:18). As the most important of them, being ancestor to King Saul, and his great captain Abner (1Sa 14:50), the last named, Abiah, was literally Becher's son, it would seem that the rest (with others not there named) were likewise