-how many modules can be supported by 1 power module (including weapon turrets),
As many as you can fit within range (~5K). It varies because some of the modules are different sizes. And you'll also want to space weapon turrets out a bit so they aren't obstructed by other nearby modules.
-how many modules can be supported by 1 crew quarters module,
Again, as many as you can fit within range for the modules that depend on them. If a build dependency is not met for a crew condition, the game will generally let you know before you build so you can place a crew module, then continue.
-the same question about storage module?
Does positioning of shield and power modules really matter except cosmetics purposes?
It can. You can shelter them from attack better by building around them, stack them together for added redundant benefit, and place them in a central location with dependent modules around them for a more efficient and/or compact design.