If Biden got 80 million votes and Trump 74  what is the total number of registered voters in the USA ?

It's not always only a third.  It's usually higher in Presidential elections than in midterm elections.  In most recent Presidential elections, it was about 50% (1/2), but it was about 60% (3/5) in 2008 and 2016.  In 2020, there were about 240 million eligible to vote, and about 2/3 did vote.  This is actually a record for percentage or fraction of all adults, even though it was over 80% (4/5) of all eligible voters at one time, because that was when women weren't eligible to vote, so 80% (4/5) of all eligible voters meant 80% (4/5) of adult males, or about 40% (2/5) of all adults.

There were 240 million people eligible to vote. A record 224 million registered.

Not all registered voters did vote..

I feel bad for you.

I really do.

Your guy lost.

One of these days you'll accept reality.

About 68% of registered voters, voted in 2020.

I'll let you do the math to find a total number of registered voters.