A few times I’ve forgotten to write in any plate numbers, and have never been cited. But we don’t often visit busy parks in the high season. If you want to game the system, you could leave it blank and hope the ranger would let you write in at the time (if caught in-person). That said, if you can afford it, it’s $35 to buy a second pass, and the parks can use the $$. We currently have three vehicles so have two passes.