ANSWER:

You should make it up if you had a legitimate reason such as illness or traveling. You should also make it up if you started fasting a day or more of Ramadan and then broke that day without completing it.  

If you didn’t fast Ramadan for no legitimate reason and you didn’t break your fast but rather did not fast that day from the beginning, there is no making up upon you and you must repent and ask Allah for forgiveness. 

I reverted last Ramadan but couldn’t fast then , So I am making up my fasts for last year . And i read somewhere that we can have double intention on a day of fasting For eg- ramadan qaza fast + monday sunnah fasting if i’m fasting Monday But now i came to know we cannot do that. Should i repent and that’s enough or do i need to redo so many of those fasts ?