more than necessary

1a. I bought more than was necessary.
1b. I bought more than necessary.
2a. I bought it more than was necessary.
2b. I bought them more than were necessary.
2c. I bought it/them more than necessary.
Please correct the above and comment.

1a and 1b are correct. The others don’t work.

Could you please explain the reasons why #2a and above wouldn’t work?

I’m assuming that ‘it’ refers to the things you bought.
The pattern would be:
I bought more of it than was necessary.