[bobcowan]

The more cylinders you can monitor, the better off you'll be. But, for a lot of applications, it's just not reasonable.

Some systems (like Megasquirt) will allow 8 sensors. But most systems will not allow more than 2. Some people have placed the sensor in a single header pipe, and it will work just fine.

The risk is that if you have a problem with that one cylinder, it will effect the entire system. Of if you have problems with a different cylinder, you won't know until something bad happens. But those situations are pretty rare.

Some sensors need to be fairly close to the head, like 18" or so. If yours is one of them, then you'll need to place it in a single pipe.