Start with 2 rabbits, each pair has 4 babies per month.

After 12 months, you will have 12,288 bunnies.

Showing my work:

1st month, 6 bunnies

2nd month, 12 bunnies

3rd month, 24 bunnies

4th month, 48 bunnies

5th month, 96 bunnies

6th month, 192 bunnies

7th month, 384 bunnies

8th month, 768 bunnies

9th month, 1,536 bunnies

10th month, 3,072 bunnies

11th month, 6,144 bunnies

12 month, 12,288 bunnies