1. The last number of concert was impressive.

2. The last number of the concert was impressive.
(Which one is correct? Do we have to use 'concert' or 'the concert'?)

3. Everyone was deeply impressed by the last number on the program.

4. Everyone was deeply impressed by the last number of the program.

(Which one is right, 'on the progtam' or 'of the program'?)

5. He had never seen such a big crowd.
6. He had seen the big crowd for the first time.
7. He saw the big crowd for the first time.
(Does #5 mean #6 or #7?)

2 and 3 are correct.

5 and 6 mean approximately the same thing, but 7 is not quite the same because of the verb tense. It's almost the same, but not quite.