OP: it depends. Check with your local jurisdiction regards to which building codes and electrical codes are effective in your local area, then head off to the library and read up on the subject.
Some codes require limiting residential single family home circuits that contain combination lighting (luminare or light fixtures) and plug and cord receptacles to 1440 VA and 15 amp circuits with exceptions only to dedicated bathroom 20 amp circuits that may also contain a light circuit for that bathroom only and limit its power consumption. The reason for this is that most luminaires (light fixtures) by UL ratings use lesser gauged wiring than can be safely attached to a 20 amp circuit.
strict receptacle ONLY circuits do not have such limitations regards to residental wiring generally, in the codes, excepting certain areas which are specified.
Finally not all areas even ALLOW the use of cabling, so go with the wiring methods that are allowable in YOUR JURISDICTION. (Jarrod some homes and areas are 2-wire GROUNDED with THHN or TW and EMT, SHEESH!).
Determine your wire gauge by the AMPs of your circuit, derated for Ambient temperatures, Ground (EARTH) temperature, length of circuit and derate for conduit/raceways. Also adjust your AWG rating via the rating of your cabling/WIRE temperature VERSUS the rating of your fuse box/circuit breaker box terminals (i.e. if your wire is 90-degree temp rated but your terminals in your circuit breaker box and/or the circuit breakers themselves are rated at 75-degrees or 70-degrees C). you have to derate your wire for heat sink.
The 12 AWG 20 Amp and 14 AWG is a STARTING point. One has to adjust and consider MANY things not just throw in what some poster with a cheat guide and no clear understanding of the codes are. Many jurisdictions in the US use ICC and others use NEC (NFPA Sec 70) for their basis for electrical codes, and they adopt a particular "edition" or Version date for their code authority. Oftentimes local jurisdictions will make modifications to that published code, adding or deleting requirements allowed in their local area. Please check with your AHJ (Authority having jurisdiction) before begining your project and plan accordingly.
Once you know what "code" you need to follow, educated guidance can be offered.
Jarrod's "pat" answer is NOT NECESSARILY CORRECT, as it was not properly "qualified" regards to both NEC and ICC not allowing combination circuits involving luminaires at 20 amps UNLESS EXCEPTED in residential circuits.
Furthermore, if this is other-than a single family dwelling you need to qualify that, as certain attached dwellings (like a 3-flat condo for example where the lower flat includes multi-levels, e.g. basement), a multi-family dwelling, etc. have additional wiring restrictions/considerations.