Is my steem power changing from how many upvotes I give?
I heard somewhere, I can upvote only 20 time in day is that true?

First, your steem power's total amount never changes no matter how many time you vote.

But, your power rate(per day) will decrease as you vote to someone.

Second, 20 time in day ? absolutely no. You can vote hundred times and more than that XD

But, as I mentioned, as you vote, your daily voting power will decrease,

For example, if your 100% voting power is 1$, next time you vote will decrease to 0.9$