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.
Four boards at 5 1/4" will give you the 21" you are looking for. Any trim will give you an over hang and a finger hold to open the chest. Try to match the grain up as Glen pointed out and alternate the crown of the boards to help eliminate/help with wood movement.
It would be great to see some pics of your hope chest.