Bug with the repair of machine parts

In the engine 8m spark plugs: 6 pcs. 70% and 2 pcs. 5% each.
For example: in the engine it is necessary to replace 2 spark plugs out of 8 pcs., I buy 2 spark plugs to replace, I insert 8 pcs. into the engine, and in the list of the state of the car it is marked in red as but one candle is missing. I buy 8 new spark plugs, replace them and everything is fine. But it's not profitable, the money goes nowhere.
I noticed that they need to be put strictly in their place in the sense:
If out of 8 cells in the 1st and 3rd you need to replace the spark plug, then you need to replace them, and not the 2nd 4th 5th 6th 7th 8th. That is, if I replace the 1st and 2nd with new candles and 345678ya remains with the previous good candles, it will be noted that one candle is not enough. The problem is that when you repair the entire car, you not only need to replace the spark plug, but also the “valve rocker”. "Connecting rod caps" "piston with connecting rod" and many more such parts, and it becomes simply unbearable, help plz!