If there are more negative counters than positive counters, what will the sign of the sum be?

Negative. If a negative number is "larger" (further away from zero) than a positive number, the result will be negative. You're basically subtracting a value that is greater than your starting value.

For example:
8 positive counters plus 18 negative counters:

This can be expressed as 8-18, which equals -10. The sign of the sum is therefore negative.