Your cat ate about 5 times the therapeutic dose! I see your question was posted a few hours ago, so I am hoping she is either sleeping quietly and OK or you have taken her to your vet if she is having difficulty.
The following is the info from plumbs veterinary drug handbook about overdosage of diazepam..........
"When administered alone, diazepam overdoses are generally limited to
significant CNS depression (confusion, coma, decreased reflexes, etc).
Hypotension, respiratory depression, and cardiac arrest have been
reported in human patients, but apparently are quite rare."
The normal way we would treat these cases are to make them vomit if they ingested it within an hour ( we are too late at this point!) and support them through this with iv fluids if their blood pressure drops too low, and respiratory support if their breathing isn't good. There is an antidote but I have never seen it carried in a veterinary practice ( Vastly expensive and rarely used) Sometimes we will give charcoal and a special medication to decrease absorption from their intestines and speed the excretion of it.
If she is showing any severe difficulty your vet should see her