I think maybe you are confusing SEER, which is an efficiency-related rating (higher SEER means more effective use of the electric power used to compress the gas), and TONS rating, which is a measure of the total cooling power of the system. (Tons used to mean how many tons per hour of ice was used in evaporative cooling building systems - a Refrigeration or Cooling Ton equals 12,000 Btu/Hour of energy exchange. A BTU, which is another antique measure but still used, is the energy needed to change the temperature of a pound of water one degree fahrenheit. Unfortunately, because of varying humidity and evaporation, this is not readily related to house air heating or cooling without a lot of assumptions and some computations.
Relating this to today's world - the Manuals BayAreaAc referred to account for all these energy conversions and determine an estimated cooling (or heating) requirement for your specific house. The type of construction, solar exposure, general climatic conditions such as average temperatures, humidity, and hottest and coldest normal ambient temperatures and desired inside temperture are all taken into account in the more sophisticated versions of the analysis, so there is no "standard", though a rough old rule of thumb was about 1 ton of cooling per 500SF of house. Obviously, this was a WAG only because it did not account for insulation, type of roofing, whether you live in Alaska or Miami, etc. The ACCA manuals do a very simplified form of evaluation to arrive at a "design", which generally will be adequate. OF course, highly precise calculations are not really needed because A/C units generally come in even ton ratings - so if you are at say 2.6 ton requirement you will be getting a 3-ton unit anyway.
SEER ratings are not a direct measure of efficiency, but the relative difference between ratings gives you good idea of the unit's relative efficency in using electricity - so a 16 SEER should be about 19% more efficient (so roughly comparable lower electricity bill) than a comparably sized 13 SEER unit. 13 SEER is the lowest efficiency currently allowed to be built for general use, 19 SEER is about the highest efficiency made by pretty much all manufacturers, and about 25 SEER is the highest rated though very pricey shelf-item units, though special construction custom units can reach about 30 SEER.
Note however, like any government sponsored rating, much of it is hooey when you get down to it - for instance, SEER ratings are figured based on 80 degree inside air temp and 82 degrees outside, when that is far from the normal case of mid to low seventies inside and high eighties or above outdoors. This makes the absolute SEER rating meaningless, but relative numbers still have meaning in comparing units.
Note these efficiency ratings are for conventional air conditoners and heat pumps working in ambient air conditions. Ground sourced Geothermal or lake/river exchange cooling units, though initially more expensive in most cases, can greatly exceed the air-exchange unit efficiencies because they are exchanging heat with cold natural water rather than with a high-temperature outside air, and instead of continually compressing a gas are just circulating cold water. I worked on one geothermal cooling project which had almost infinite efficiency, which of course makes no sense - but the only power was for sensors and a control valve as the water flow was single-pass under gravity flow, so no power was used to circulate the water.