The measurements you want are not shown. You need the inside width of your base cabinet and the exterior width of the sink bowl. The measurement of 20-7/8" is to the outside of the rim. I'd guess it's very likely to fit without any problem. But you have to measure. A strap can be used to secure it if necessary. Hercules Universal Sink Harness Kit - Amazon.com Joe may have better info. Totally adequate size kitchen sink, imo.

That sink should undermount in a 21" cabinet with no problems. Install a strap system to the cabinet sides, cut the cabinet sides down an inch or so, set the sink flange on the cabinet sides, install the top, silicone the sink flange, clamp the sink, check your reveals, strap the sink and pull the clamp.

^^^ well said above by Joseph. The nice thing about the size is that the sink flange has such a firm mount as it's setting ON the cabinets, not "hanging" from the counter.

WARNING - this will be VERY tight for your door hardware. It may negate having a full height door, or you may have to move the hinges below the sink because you just can't get any hardware between the sink and the side of the cabinet. (ask me how I know - d'oh!)