The Rucker 4.0 20L (both versions) can hold 1 plate securely. One Rucker 20lb or 30lb plate can fit in the elevated velcro plate pocket (the GR1 versions will not fit).
There is still room left in the interior if you wish to add more plates but they will not remain as secure and may shift some while rucking.
If you're looking to beef up the weight, the Rucker 25L and Long Range Ruckers come with two plate pockets that can hold a 45lb plate in the back pocket and either a 20 or 30lb Rucker plate in the front pocket. Keep in mind, the Rucker 25L and lONG range Ruckers are longer rucks, so if you are under 6', you may find that the bottom panel rests lower on your back than you would prefer with the 20L.