i would definetly start by bulding up ranges that are approved in a vacuum (e.g. that play out well vs an unknown player).
Its kind of fundamental to have a startingpoiint in an unknown environement.
That counts for everything: openingranges, 3bet vs specific positions and hands to 4bet+ with, depending on the strategy maybe flattingranges as well. There is plenty of charts to find on the internet that give a good start (cant remember if peter added some to his course as well).
However we shouldnt work static with these charts. Winning at lower stakes is mostly a thing of exploiting weaker opponents and the hands we start with are kind of the 1st step to start with exploitation (like stealing more than your chart vs nits or vice versa folding more than it if a nit opens).
Not quite sure what kind of counting combos you mean. I guess a % of range will help more preflop than knowing that we open 325 combos from the button.
But if you mean counting combos postflop when facing a difficult decision i would recommend some training on this. Having a clue how much combos you and villain might have (or not) in a certain spot is very important to justify calls or folds given a specific price (e.g. hitting the minimum defens frequency=MDF).

I think you need to use your preflop ranges as a starting point (you can adjust these slightly depending on opponent types). Having a good understanding of these will help you picture which hands are in your range on the flop, and how you would play these given the flop. This will help you recognise when to be polarised or when to merge your range. In game (especially online), I don't think top players are counting all the combos of various hand classes in their range or villain's range, but having practised and analysed it off the felt, they have a good knowledge of roughly how many bluffs are available vs how many value bets etc