Charles Ellicott Commentary Joshua 4:8-9

"And the children of Israel did so as Joshua commanded, and took up twelve stones out of the midst of the Jordan, as Jehovah spake unto Joshua, according to the number of the tribes of the children of Israel; and they carried them over with them unto the place where they lodged, and laid them down there. And Joshua set up twelve stones in the midst of the Jordan, in the place where the feet of the priests that bare the ark of the covenant stood: and they are there unto this day." — Joshua 4:8-9 (ASV)

According to the number of the tribes... .— Every tribe was represented by a stone on either side of the Jordan. The two cairns represent a complete Israel in the wilderness, and a complete Israel in the promised land. Thou shalt remember all the way that the Lord thy God led thee. By the grace of God I am what I am.