how many games does it takes to make your elo accurate

If you read the material from Mr. Glickman, the Boston University Professor  who invented the rating system used by Chess.com it will never be 100% accurate.  The best you can hope for is to get it within a minimal statistical error range.  https://www.chess.com/article/view/chess-ratings---how-they-work

In general, the more you play, the more accurate it will get.  You have played 82 rapid games recently. Your 851 rapid rating is likely pretty accurate. 

However, you played only 2 daily games. It is likely that your 742 daily rating is less accurate.

A dozen or so games should be enough to get your RD (Rating Deviation) down to single digits, as long as those dozen games are all played at the same time-control.

Just a FYI: the rating system used on this site is not Elo. It's Glicko-2.

I have 1,420 blitz games and still have a double digit RD value.

It also depends on the time frame in which the games are played, since RD grows with time and shrinks with games played.

+1

So is the lower the number of RD the better you are or the worser you are?

I don't believe a week ago is enough to raise it that much, though I could be wrong.

RD is a measure of how accurate your rating is estimated to be. It has no bearing on strength estimates beyond that.

I don't think I've ever seen a single digit RD, and I've seen people who've played 40 games a day, almost every day.

In extreme cases it might take something like 100 games to get an accurate rating... let's say your friend plays 100 games and raises / lowers your rating far from where it should be.

But normally, like others have said, a few dozen will make it really close.