On a 21" wide panel I prefer my boards to be 6" or less. Having said that it's just my preference.
There isn't any set rule for all the boards being the same width. I think 3 boards is easier to get a nicer grain match across the panel tho. 4 boards in my opinion would be the most I use for that size.
A side note would be to wrap a piece of 3/4 Oak plywood with solid Oak & eliminate movement issues & grain alignment issues. However Oak veneers rarely give any figure pop if you're looking for that.