AC outlet

A standard AC outlet is usually 15A, or up to 1,800w if you have a solid 120v incoming. Keep in mind that you're outlet is likely sharing a circuit with other outlets nearby, all drawing from the same 15A. That being said, considering the things you listed which are currently plugged in you likely won't be drawing any more than 600 watts at the moment, less than half of that if your PC isn't an extreme gaming rig. You can add many external hard drives without having to worry.. They probably use between 5-10w each.

Depends on which circuit breaker that particular outlet is connected to. Some houses have multiple outlets or rooms on a single circuit breaker. Current electrical code calls for 15 Amps, but older houses can have 20 Amps. Once you know what else is on that circuit and the rating of the breaker you can safely figure out how much total you can do. To find out what your devices max draw is they should be labeled with an amperage amount or at least a wattage which you can use to figure the amperage using ohms law.

Most of the devices you want can safely be run off a single receptacle. Also best to use a power strip with a surge protector with the wattage rating you need instead of outlet multiplier and don't daisy chain a power strip off of another power strip. Basically just keep the Toaster oven and Microwave on another circuit and you should be good.

You'll probably be fine adding an external drive. I have like eight devices on my power strip (PC, Laptop, Xbox 360, Monitor, Speakers, Printer, Modem, Router). Just make sure you have a good power strip with surge protection and not some cheap piece of crap.

If in any serious doubt consult a qualified electrician. Any half decent electrician would happily give you the advice you need without charging you.