shots does it take to foul a barrel after you clean it to get it back to the original sight in point?

well that depends. i could take none, it could take 10. it could take 2.
one variable is how many shots you took to sight it in in the first place.

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If the barrel is clean with no oil in it, my guns are usually right on with no fouling shots.
I have owned rifles however that required a couple fouling shots. If your barrel is reasonably clean with no oil in it 2 shots should be sufficient to get back to your zero. You need to check your zero often as it may shift for various other reasons.

You can test the need for fouling shots at the range for your rifles.

Just in case after all my sighting in is done, I always clean my barrels good right before the season and then shoot a couple rounds out of a clean barrel before I go hunting. Then don' t clean my barrels until the season is over unless the rifle gets wet.