I followed The Pec Shef's hummus recipe and I am getting a strong horseradish after-taste in my hummus. I only used 6 cans of chickpeas instead of 8, and a 9.25oz jar of horseradish. Is there anything I can add to the mix to reduce or neutralize the strong horseradish taste?
Yes. Make it again and don't add horseradish. I'm totally serious - no traditional hummus recipe in the known universe has horseradish in it. There is nothing you are going to be able to do to your existing batch to remove that flavor, other than diluting it, but I don't think you'd be able to dilute it enough to be worth the effort.
One way you could reduce the potency of the taste is by adding more chickpeas, but that assumes you like the taste of horseradish and just want it to be more subtle. (It also assumes you have a more reasonably sized batch that doesn't already have a whole jar of horseradish in it.)
Once you're using more than one or two cans of chickpeas, you should consider buying a bag of dried chickpeas and making them from scratch. A typical sized bag (16oz) makes a lot of chickpeas, they taste better, and it costs less money.
Third, when adding ingredients with a strong flavor, or a flavor that could find disagreeable if having too much, add a little bit at a time and taste it as you go. This is a good rule to follow with any dish, especially when salting.
If you want to try horseradish again, just make the hummus without adding the horseradish. Then when you have a meal, set aside a portion of the hummus and add a little horseradish to taste.
A suggestion I'd make next time, is using a few cloves of garlic (raw) instead of horseradish.
Last, maybe it's opinion, but this recipe is ridiculous. I'd make a serving size this larger if I were having a party. Be prepared to have nothing but hummus for every meal for the next week if you want to use it up before it goes bad. If you are going through the trouble of making hummus, unlike "The Pec Chef", you should also try to use as many fresh ingredients as possible. No bottled lemon juice, no canned chickpeas.