I have seen quite few positions where opponent has more than 10 good moves if I play a specific move. Even at that time, I have seen people calculate for so long. Is it that they are looking at all the moves and variation of what can happen after that? It's completely uncertain at some times what my opponent can play on the next move. Like in opening, who doesn't know theory they always look at something, but what do they actually think (or I have think to mastery on positional concept)?
There are many positions where there are a lot of good moves. 10 seems like a lot, but say that you have that many candidate moves. That doesn't mean that they're all as good as each other. In fact unless there are transpositions it is likely that there are going to be significant differences between moves.
This could be hidden tactical shots (the longer the game, the more time you should spend looking for these), or simply subtle differences in the pawn structure and the position of the pieces. A strong player might spend a long time in this sort of position as these factors become more and more significant and avoiding small weaknesses can become almost as important as avoiding losing a piece.
Another reason for spending a long time in such a position could be to decide on a plan, as although in strategical positions with a lot of moves there may be too many variations to full calculate, especially in closed positions there will often be a certain set of things you want to achieve: pawn pushes, piece manoevers etc. Deciding on where you want your pieces and how you are going to get them there will likely form part of the decision process.
There can be lots of variation in some positions as stated above. So what exactly one should calculate? I have found the same question in Daniel Naroditsky's book.
A question arises, "What exactly do you calculate?"
In other words, in a very complex position, how do you choose when to stop your calculation? The answer is simple: calculate as much as you can without getting into bad time trouble. The more you calculate, the higher chances of playing the correct move will be. ~ Mastering the position chess, Daniel Naroditsky. - page no. 47 chapter. 2