How many programmers does an indie games usually have?
Depends on the game, there is no good "usually." Even the definition of "indie game" is a bit varied, but generally you can expect anywhere from one to a small handful of developers on such projects.Are these roles descriptions correct?
Definitions of roles are vary widely from project to project or studio to studio. The descriptions you've provided seem reasonable enough for a *specific* game but aren't necessarily broad enough to apply industry wide. They're more like narrow examples than definitions.
As he said. You gave right examples but it does not define precisely the different roles. For more info, google and wikipedia are your friends (https://en.wikipedia.org/wiki/Game_programmer, https://en.wikipedia.org/wiki/Game_artist, https://en.wikipedia.org/wiki/Level_design#Level_designer)