How many people actually believe 2020 begins a new decade?

If it isn't a majority, it is very close to one.

Any part that starts or ends a decade is a decade. I have lived through 9 of them now. 1940's, 50s, 60s, 70s, 80s, 90s, 2000s, 2010s, 2020s.

Me and many people.

Everyone but you dummie!

I thought 2020 was the start of a new millennium 

It does begin a new decade. It is not a part of the 2010's.

Most people classify the decades this way, few care about 1AD and 1BC

99% of people..

2020 does begin a new decade.

YES OF COURSE IT DOES...