I don't disagree with the above, but also don't want to generalize. I'm not sure what OP's actual products/product categories are - so it should actually be risk assessed as to whether one pre-op per day is appropriate. I know we tend to throw around the phrase "risk assessment" a lot in the Quality world, but I feel this warrants it. If justifications are reviewed and documented, then one pre-op should suffice with the line running continuously. Higher-risk product categories might require more than one pre-op. It could also very well depend on the nature of the product - for example: does it harden or clog up the production equipment over time and require some kind of intervention periodically during the shift?
In my opinion (not saying I'm right), I would prefer to have one per shift. It keeps people acclimated to conducting proper checks, and also gives a bit more assurance that the line was clean, in good standing, and setup correctly. I'd personally like to have to put less product on hold rather than more if something is found to suggest an issue.
Ultimately though, as long as the written risk assessment justifies the reasoning, one pre-op is fine.