Today I read a news which is, death toll maybe 300 "and" 400.
Here I think "to" is correct.
Yes, I believe that you are correct. It could also be written as, "...death toll is 300 or 400." Your text is the best if the number is between 300 and 400. You could use "or" if you are unsure of whether the number is 300 or if the number is 400.