There  is no measurement for writing the testcases avg for 
a day. That depends on functionality. If the functionality 
is critical we can write just 5 to 10 test cases otherwise 
we can write 60 to 70 . In this way the average for day is 
30-35.

in my company i can write dialy 20-30

its depends on functionality of the application

It depends upon company to company. 
If ur covering flow level test case, it may have many test 
steps so u can write 5 to 10 per day

Or 

If ur covering field level test case (Range, size, BVA, ECP 
etc.) count will be more.

and also the area ur covering and ur understanding of 
requirement.

It is depend on application

Testers are supposed to be talkative. In order to keep their
point floating they have to argue, give reasons for everything.
Hence such questions are asked to measure the capability of
a tester to argue. If u say 5 to 10 then justify your answer
or if you said 5-60 then also you have to justify your
answer. So, in all whatever you say or do be ready with the
supporting  documents.

Never answer that i can write this many tes cases per day.

The answer should be, i cant say the count, i can write the 
test cases to cover and satisfy the given requirement. The 
count of test cases per day depends on the requirement and 
its complexity 

Thank you,

Rasul
QA Engineer
rasul_Cse@yahoo.com

depends on Req and functionality.

critical T.C-8 to 10
medium-15 to 20
low-25 to 30