Welcome!
1U is an acceptable starting dose when on a dry, high carb diet. If you plan to transition to wet LC, please let us know. It must be done slowly and carefully, as it can have a significant impact on BG. As others mentioned, we take comfort in home testing and having data before making any recommendations.
They can be higher at the vet from stress, but 500 is still very high.
If there's no history of ketones, I would wait a day or two until you can get a meter and strips, and hypo kit. That way first day you give insulin you can test before the shots, and again about 4 hours after. If you're home Saturday, that's perfect.
We'll need some other things to be able to help efficiently, like a signature, etc. You can find everything you need here:
https://www.felinediabetes.com/FDMB/threads/new-how-you-can-help-us-help-you.216696/
You'll want a hypo kit - someone above provided the food list, you'll want a 3-4 cans of food 10-15% carbs, 3-4 cans 16-20%, and maybe 2 cans of something 23% and higher...as well as some karo or honey.
Home testing tips. Some vets say you have to get a pet meter like an AlphaTrak, it is not necessary. The majority of us here use the cheap Walmart human meters and they work just fine. You will still want one as a backup, the Libre (sensor attached to cat) can read artificially low at low numbers, so we recommend double checking low numbers with a handheld.
The other thing you'll want is urine ketone strips.
Diabetic ketoacidosis (DKA) is a very serious and potentially life threatening condition for diabetics and requires hospitalization. The "recipe" for it is not enough insulin (as in a newly diagnosed/unregulated cat) + inappetence + infection or systemic stress. So you'll want to test daily for urine ketones.