Think about this: What's a laptop? What's an iPad?
You want as many contexts as you *need*, but as few as you can get by with. Too many contexts causes organizing to become difficult.
A laptop is a computer. An iPad is a computer. I suggest you create @Computer for anything that requires a computer and @Computer-Web for anything that requires an Internet connection if your @Computer list regularly shows a bunch of undoable things because you don't have Web access. I'll bet that the majority of your computer-related tasks could be done on any computer. Don't create @iPad unless you're regularly encountering actions that require that device and nothing else.
The way to tell if you really need a context is to try to do without it and see if items show up on another list that you regularly can't do. That's how David Allen came up with @Computer-Web. He'd be looking at his @Computer list on a plane in the days before in-flight WiFi and see a bunch of actions he couldn't do. So he created @Computer-Web and moved all actions requiring a Web connection there. When in flight he'd just fold away that list and it wouldn't bug him at all.
Some applications like MS Outlook allow you assign multiple categories to an item. Use that capability to assign multiple contexts to a single item if that fits your situation.