Question 56913
Suppose there are 2 bats that can see with their both eyes.

This means you already got 2 bats who can see with left and 2 who can see with right.

Suppose there is 1 bat with a good left eye, but a bad right eye.

We now have: 

2 bats who can see out of their right eye
3 bats who can see out of their left eye
1 bat who cannot see out of his right eye

Now suppose there are 4 bats who are entirely blind.

We now have: 

2 bats who can see out of their right eye
3 bats who can see out of their left eye
1 + 4 bats who cannot see out of their right eye
4  bats who cannot see out of their left eye

This means the minimum of bats is 7.