Why can people only answer 20 questions a day on yahoo?

Because those people are only at user level 1, so limiting their ability to post reduces the amount of spam/chat/nonsense they can post before any violations are reported and removed. 

Learn about Points & Levels: /info/scoring_system

Because that's the Limit for Level 1 users Yahoo has a Limit on the Number of Questions that can be Answered Each Day for Each Level to Prevent Spam and Misuse on this Site

Hello Kayla...

As you are new on this it is worthwhile having a look around the site. Here is the bit about points and levels

https://uk.answers.yahoo.com/info/scoring_system

There are a lot of good contributors on this site, so please don't be put off by the trolling and silliness by a few.

:)>

because you are still new. establish yourself as a trustworthy person. then you can answer 100 like me.

Limiting newbies to 20 answers a day is meant to stop them and spammers from flooding the site with things the site isn't for

I can answer 160 questions a day. 

It all depends on what level you are on.  As a level 1, you can only answer 20 questions per Yahoo day.  Once you move up a level it increases.  At level 2 you can answer 80 questions, level 3 you can answer 120 questions, and level 4 to 7 you can answer 160 questions per day.  Please read the points and levels on the bottom left in the subject  "About" for a lot more information.   The more points you earn the faster you will move up a level.

That's ONLY for level 1 accounts. The more questions you answer  for 2 points each, and the more best answers you choose after at least 1 hour or 1 answer, for 6 points each, the more points you have to ask more questions per day. When you have 250 to 999 points you will be able to answer 80 questions per Yahoo "day."  

/info/scoring_system...  

It depends on the level you are in. We who are at level 7 can answer 160 questions per day.