There are 2 unknowns here. One, the number of eclairs, Two, number of friends he shared with.
Method 1: trial & error

Suppose he has 5 friends:
When he gives 8 eclairs each, he still needs12 eclairs.
This means: 5x8= 40 eclairs needed
Number of eclairs he has = 40-12=28

If 4 eclairs given out:
5x4=20 , but he has 4 more eclairs left.
This means number of eclairs he has Originally
= 20+4 = 24

Since number of eclairs does not tally, it means he didn’t share with 5 friends.

Let’s try with 4 friends:

8 eclairs each= 8x4 = 32 eclairs needed
But original number of eclairs he has
= 32-12=20

4 eclairs each= 4x4= 16
Add the 4 remaining = 16+4 = 20

Number of original eclairs tally, this means he has 20 eclairs originally and 4 friends that he shared with.

Now, question asks if he gives each friend 6 eclairs, how many more Eclairs he needs.
So, 6 eclairs each = 6 x 4 = 24
Since he only has 20 eclairs originally, he needs:
24-20 = 4 more eclairs