Hello,
As you have completed your bachelor's in the duration 13-16 but really passed it in 2018, so you are eligible for m.sc maths. But it also depends on the eligibility criteria for different colleges. I guess you took two years to clear your backlogs.
Also please let me know the name of the college where eyou want to take admission in PG Maths. Once o get that, I can help you with all the details like eligiblity criteria, fees , admission process, etc.
Please feel free to reply back
Hope it helps