Hi exploring
That should be pretty straight forward for you if you create all 1000 questions in one single block and via block options enable question randomization and select 3rd option which reads "Present only X of total question" which in your case X = 20.
Please ensure that you don't have any timing variables in this block as that would be also counted as one of the 1000 questions.
If you have a requirement to further funnel this down to select randomly X questions from a pool of Y questions - you can further break this block into multiple blocks and use the same setting to present the required number of questions.
Let me know if that answers your question.

Thanks a lot for your great response, this is fantastic.

I have to also put a timer of 30 mins for the complete survey i.e. to complete the 20 questions.

So are you saying with the above mentioned approach I cannot leverage timer?

Also I have not quite followed what you meant by the following "If you have a requirement to further funnel this down to select randomly X questions from a pool of Y questions - you can further break this block into multiple blocks and use the same setting to present the required number of questions."

Hi exploring
You can program you timer logic in the survey flow and keep that out side the block of 1000 question. My other comment ""If you have a requirement to further funnel this down to select randomly X questions from a pool of Y questions - you can further break this block into multiple blocks and use the same setting to present the required number of questions" was meant in case you have 50 questionnaires where you want to ensure that you get one question from each questionnaire then in that case you will need to create multiple blocks and randomly ask 1 question from each block.

Thanks again for your fantastic response. One more question from my end.

Suppose I have to treat 2 question as a logical question. Is is possible to design block as such. I.e. out of the 1000 questions actually I have 1000/2=500 questions.
As for a respose user needs to fill a text box and a multiple choice and both of them are actually one logical response from my side.

Question 1a: Can you rank X useful between 1 to 5?
Question 1b: Can you write down why X is not useful?

So I want to treat 1a and 1b as one question.

I believe you can use in-page setting for the follow-up question for your requirement but I am not sure if system will treat that as one question. Will need to try that out.
I will try to create one example if I get a chance today and will let you know how that works.