Extension cords need to be chosen according to the length and load. The following chart will guide you to the correct gauge to choose for any one extension cord. You might choose several different gauges in order to save money, properly configured you won't need to put 12awg wire everywhere:
To carry 20 amps any distance, you'll need 12awg cable. You could use one 50 foot 12awg cable to carry the current to a table, then three 16awg cables to carry the current to one or two computers each.
You might find it cheaper, however, to run two 14awg wires in parallel from one outlet than to run one 12awg wire.
Keep in mind that while some cable calculators and tables will indicate that 16awg can handle 20A, the cable will heat up, and if you've properly secured it to the floor using tape or channels with other cables running parallel, the heat buildup can be significant and dangerous. Particularly with long runs.
You should also consult with the hotel - many of them have requirements for extension cord use in conference areas, and in some cases you will be required to use their cables and installers in order to meet their safety and load planning requirements.
If you pay attention to the chart you'll note that each cable size and load include a voltage drop. You'll also realize that due to the wiring at the venue you'll see a voltage drop at the outlet as well. This will result in a lower voltage at each computer than 120V. However, as long as you keep your voltage drop below 10%, and the venue is wired correctly producing a less than 10% drop to the outlet under load, then it is unlikely that your computers will experience any problems. Most computing equipment that uses AC power has a universal power supply that accepts a wide range of voltage, often down to 90VAC. It will be a rare computer or peripheral that will have problems with the voltage drop presented by a suitably rated cable.