TBF they probably shot at more than 6 but less than number of detected/locked aircrafts. Some of the missiles clearly did not score hits, evidenced by the stray PL15Es on the ground.
BVR is voodoo magic anyways and there's still no mutually agreed upon way to conduct it. Basically every air force has to tailor their BVR doctrine significantly with regards to their capabilities and the situation they're given. In this case, it is yet to be seen how the whole thing played out turn by turn and how jets on both sides played the whole thing, how many times did they try to attack, how many times did they try to defend, what was the jamming like, how much did the associated assets like AWACS helped.
It seems that whether it be by better tactics, by RoE or by whatever other factor PAF had the significant upper edge but that doesn't mean that they had the easiest time shooting at the Indians. Given the conditions, a kill might have required anything from 2 to 6 shots per kill if not more. Add in the terrain, the realisation by the Indians that they're being shot at, the jamming, the WEZs, what the pilots had for dinner and whatever other factor that you can add in that even despite locking on to 20 jets, 6 was the best that PAF could get that night.
It might be so that at the end of it all, 6 or so was the number that PAF could cautiously kill without putting its jets in danger of being shot down. It might also be so that only 6 were in the good WEZ of the PL-15, it may be so that taking more kills might have meant pulling jets from defensive roles into offensive roles, it might be because of fuel, it might be that there were only so many J-10s to go around and the PAF didn't want to risk putting in JF-17s and Vipers for risk of loss and diplomatic complications respectively.