jmc

The least positive integer which is congruent to 6 (mod 11) is 6. The other positive integers which are congruent to 6 (mod 11) are $6+11$, $6+22$, $6+33$, and so on. We seek the maximum positive integer $k$ for which $6+11k<1000$. This maximal $k$ is the greatest integer less than $\frac{1000-6}{11}$, which is 90. So the set of positive integers less than 1000 which are congruent to 6 (mod 11) is $$\{11(0)+6,11(1)+6,11(2)+6,\dots ,11(90)+6\},$$and there are $\boxed{91}$ elements in this set (since there are 91 elements in the set $\{0,1,2,\dots ,90\}$).