Guns may be kept in the car if unloaded and locked up, not accessible to the driver. So in the trunk, in a case, unloaded, is fine. Behind the seat in a pickup, unloaded in a case, is fine. If you have kids, and perhaps otherwise, the cases should be locked, but that goes without saying.
If you're keeping ammo as well, it must be physically seperate from the gun. Gun in a case behind the seat, ammo in the glove box, is fine.
I don't know about loaded magazines, but they're probably okay if they're treated as other ammo, and kept physically seperate.
Hi capactity magazines are fine to possess if you bought them legally. I *think* the ban is on sale of new ones only.
No "total rounds carried" rules, as far as I know.
Note that this applies to urban areas, the rules may be looser in rural areas.
Kevin