If your gun is really a Remington "Model 11", it should hold 4 shells in the magazine and one in the chamber. If it's a Model 11 that at present only holds two in the magazine and one in the chamber, it's because somebody plugged it to comply with hunting regulations. That plug should be removable if you want to remove it, but whether you should remove it or not depends on what you want to use the gun for. If you want to hunt with it, you may need that plug to stay in the gun. Check the hunting regulations in your state.
On the other hand, if your gun is actually a Remington "Sportsman" Model - which is a sometimes incorrectly referred to as a "Model 11" or a "Model 11 Sportsman" it is actually a redesigned gun very similar to the Model 11 but with the magazine capacity permanently reduced from four shells to two; i.e. it's total capacity is permanently 2+1. This was supposedly a conservation move.
If your gun is a Model 11, MODEL 11 should be rollmarked on the Breech Bolt where you can see it when the bolt is closed.
If what you have is a Remington Sportsman, the word SPORTSMAN, or the words THE SPORTSMAN or SPORTSMAN MODEL should be rollmarked on the breech bolt.
If your gun has a plain unmarked breech bolt, you MAY have a very early version of the gun Remington was licensed to manufacture under the Browning patent. These guns were called "Remington Autoloading Shotguns" and were made under this name from 1905 through 1910. These guns had a 4+1 capacity. The Remington Autoloading Shotgun was redesigned in 1911 and was named the Model 11, to correspond with the year of the redesign.