I need to ask two questions, Can I ask this in same post or different post? These questions where based on same tags but they are entirely different.Last time when I asked a question in comment I got a reply as one questions per post. Today when I tried to post two different questions I am getting "You can only post once every 20 minutes"
One question per post and yes, you have to wait.
If you ask more than 1 question per post, how are answerers (and the community at large) supposed to handle it?
Effectively, you are just marking it harder for the community to handle your post effectively. Each question should be able to stand on its own and be judged for its own merits. Similarly, it is better for you as you know that answers will directly address your one question and not parts of your question.
All this being said, there is some flexibility for 2 very closely related questions in the same post. And I don't mean closely related in that they have the same tags, but closely related enough that the same answer can address both question. However, I would not recommend you do this unless you have enough experience to judge just how closely related the question are.
As for the time limit, that is there as a spam prevention measure. The rate limiting effectively limits how many questions you can ask in a time period and also how quickly you can ask them. You can reference The Complete Rate-Limiting Guide for everything but you are getting tripped up by the 1 question per 20 minute limit. Fortunately that is a temporary one that is lifted once you have achieved a minimum rep of 125.
However, a side effect of the rate limiting also helps guide new users in how to ask good questions. Asking a question is not supposed to be a rapid fire exercise. Do not just throw out 2 or 3 quick questions and hope that someone gives you an answer. We expect users to do their research and finally decide to ask on Stack Overflow after they have exhausted every option. And then writing a good question should take time. And usually the act of writing a question actually help your research because it might give you more thoughts on additional keywords to include in searches. Additionally, Google is your friend in these matters. You'd be surprised how many questions are easily solved by searching the exact title in Google.
Yes, only one question per question.
Also be sure to do some research before asking or your question/questions will likely get downvoted or closed.
Please familiarize yourself with the site and its rules before continuing. Otherwise there is a good chance you won't be happy with the results.