Its been discussed many times before, but there are many ways of connecting LEDs. If all of the LEDs are identical, it is possible to arrange strings of them to build up the operating voltage to a bit below the supply voltage, put several such strings in parallel, and just have one resistor. In the real world, each string would need its own resistor. The simplest way for reiability and ease of figuring out what went wrong (!) is for each LED to have its own resistor. Different colour LEDs of the same type require different current for the same light output, so you can't put different coulours in the same string (in theory, in practice it often falls into the "got away with it" category).
Having strings of LEDs cuts down the total current needed, but isn't always easy to arrange. Lots of resistors might be a pain to hide. A few resistors might start pushing their wattage rating, and that could lead to unwanted heat (says he who once nearly melted a switchboard by putting all the lines to "hold"). Small resistors, taken to their maximum, gather a lot of temperature. Swings and roundabouts, yer makes yer own sacrifices.