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In theory they followed this: http://vignette3.wikia.nocookie.net/warhammer40k/images/3/38/Space_Marine_Legion_Structure_1.jpg/revision/latest?cb=20131001033403

In practice it was a random mess, partly due to the character of each legion, partly because Black Library authors can't be bothered with boring things like TO&E when they can just throw around large numbers and words like Great and Grand.

I always took at as generally revolving around

10 squads - 1 company
10 companies - 1 chapter
10 chapters - 1 legion

obviously some of them were much bigger or had different organisation patterns but that's the basic way I have always pictured it in my head.

There were Companies, yes, but how many depended on the Legion (i.e. their numbers and particular organisation).

IIRC, the Death Guard only had 7 'Great Companies', with Nathaniel Garro the 'Battle Captain' of the 7th Company. I can't remember their supposed numbers, though.

the UM were basically a scaled up chapter, and everyone else had a basic 'outline' that was mostly ignored in favor of special companies and formations based on what the Primarch wanted to do.

I guess the easy answer to your question is: there is no legitimate answer unless you are asking about a specific legion.

Yeah, one thing worth noting is often a distinction between a 'great company' or 'grand company' and a 'line company'. The former is something far closer to a chapter or more (much lije the space wolves are today)