is it better to lean 1 opening and try to master it or learn a bit of every opening?

Well it depends on what your goals are. If you only care about rating then you should only play one opening. If you want to improve and further understand your knowledge of the game I suggest playing a variety of openings. 
If you want/need any help with that either I can help. I'm a chess coach, and my specialty is in not memorizing openings, but rather playing a variety and being creative whenever I can which includes openings.

I been playing scotch gambit from 500 to 1200. Is scotch gambit viable past 1200 rating?

Of course it is. Kasparov played it against Karpov in the 90s and did quite well overall

I recommend focusing on one main White opening, and two main defenses as Black (a defense against e4, and a defense against d4).

Against all other White openings, you can usually use a combination of basic opening principles along with your usual d4 defensive setup.

Find the repertoire that you enjoy, and aim to master it ...

One opening mastered, a hundred openings forgot

"Mastering an opening" is not a reasonable goal unless you're a GM. Or close.

I am a 1800 and I don’t know any opening except the Italian-that too till a basic level. Whenever I fall for a trap or get outplayed in the opening ,after the game I check where I went wrong and the main line. This helped gain elo slowly,but steadily. Don’t learn 1 opening. Just learn the concept of all openings and the idea behind it. Learn all traps and tricks of your main opening and trust me you’ll be fine.

I recommend focusing on one main White opening, and two main defenses as Black (a defense against e4, and a defense against d4).

Against all other White openings, you can usually use a combination of basic opening principles along with your usual d4 defensive setup.

Find the repertoire that you enjoy, and aim to master it ...

Focus on general chess principles rather than trying to memorize specific move sequences.

what is the overall plan of development? What are the weak/strong points? What is the ideal pawn break?

when I teach an opening to a student, I provide a few variations that are no more than 5 or 6 moves deep. Then I spend a half hour explaining the ideas of the opening, as in the previous paragraph. It is easier to remember these ideas than it is to memorize specific move sequences. The student can then play the opening with these general ideas in mind. Specific move sequences and traps can be picked up with experience. They will then be remembered better than if we had started with pure memorization.

A good way to know if you understand an opening is if you can explain these themes. If you can’t, then you really don’t know the opening.

When I was in high school, we had a team that played other schools every week. Our team played a lot of speed chess against one another. We also played through and analyzed all of our match games.

the four top players deliberately selected different openings to make it hard for opponents to prepare for us. Against e4, we had players who responded e5, e6, c6, and c5. Against d4, we had players who responded with the Nimzo, Kings Indian, Gruenfeld, and Benko/Benoni. As White, we played e4, c4, d4, and Nf3.

Since we analyzed all of our games, we were exposed to every opening. We may not have memorized sequences at length, but we got a good feel for how games developed out of the opening. We all reached Class A or higher by the time we graduated 2 to 3 years later.

so, I think one does not have to restrict oneself to a single opening. Focus on understanding rather than memorization.

Thank you, Mikewire.
Could you please explain some examples in detail?
May be just one idea of any opening variation, thank you.

What worked for me ... Taking an opening that had a spacefic pawn formation, treating it like "System" then learning the best lines to transpose into... then moved into playing main lines that stem of the pawn formation.

Example: I first started to play the Colle Koltanowski ... (and still do!) 9.b4 ! Against the most critical ...Nc6. But ... learning the ropes i came up against the Anti-Colle lines...which are totally playable (and fun) then it was easily to pick up Slav a Semi Slav lines as White... because imo they are better ( really the Slav and the Semi-Slav Slav what I play the most...because few want to go down Colle lines....

As soon as I played my favorite and most played opening Scotch gambit, my rating went back up from 1000 to 1200. Before I was doing a different opening which is why I lost more than 200 ratings. But Im worried the Scotch gambit isnt good in the upper ratings and can get easily exposed

I don't want to be rude, but if you are a chess coach you should know the difference between the Scotch Game and the Scotch Gambit! Kasparov never played the Scotch Gambit against Karpov, he played 4.Nxd4.

Yes, as long as you are happy to transpose to the Modern Variation of the Two Knights.

Hi! wonyoungja. 
I've play scotch gambit too and change to danish gambit and then Italian now.
Just for my little experience, I think scotch gambit is good until at least 1600.
I'm just about 1600 chess.com now.

I learnt a few lines for a few openings and tested them then study the one that worked best