We have 7 in 10 years. Current ages run from 18 to 9. All single births.
We've never left one behind. People laugh at us when we do it, but we do a head count every time we leave someplace. Even when we were fulltiming in an RV, and driving 2 vehicles, we'd radio each other on our 2-ways: "I've got 3;" "I've got 4."
When I worked nights, and DW was out with kids, she'd remind the ones still here to let me know as soon as I woke up who was with her and who was home with me.
2 stories about almost losing a kid:
When DS #3 was just a couple of months old (and he's only 13 months behind DS#2), my wife stood in the church nursery, spinning in circles telling me she couldn't find the baby, and had looked everywhere for the baby. I started laughing; she was holding him. She was so used to holding a baby on her hip for the last few months that she didn't notice it.
When DD #7 was about 2, DW took the rest of the kids to go ice skating. I was staying behind with #7. It was December in Ohio, and already after dark, and about 5° above 0. I helped get everyone in the van, then went back in from teh garage. We had told DD that they'd be back in a few hours, but kids that age have no real concept of time. I called out to her to say I was going to make dinner. No answer. Called again, a little louder. Still no answer. Checked the TV room, then the bathrooms, then closets, then tearing the house apart. The dogs are getting riled up, because they see how nervous I'm getting. Calling to her "Daddy's looking for you and it's time to stop playing," etc. Getting frantic now. Called DW. "Are you sure you don't have DD? I can't find her." She turns around and starts heading home. I go outside in the frigid air, looking for footprints in the snow, thinking she's gone out after them. No footprints. DW calls me back, asking if I've checked this room or that room. I make one more pass through the downstairs before calling 911. And found her. Curled up under teh piano, hidden by the shadows. Asleep. She had gone there to surprise them when they got home, and had fallen asleep. One of the few times I've cried honest tears of joy.
Edit: i told you all that, and I'm not sure I answered your question. We just told ourselves around #3 that we were not going to end up on CNN for losing a kid. We've just made it a point to very methodical about keeping track of them. Then again, we have full time RV friends with one kid who left him behind at a rest stop once. (He was 16, and had climbed out of the RV without them knowing it.)
Let me know if I can answer this better.