>> do you create an alias and tell it which of the 5 to drop its mail into?
Presumably, depending on the company, but that's how I'd expect it to work.
>> Can you reply from the alias with it appearing completely to come from the alias as opposed to one of the 5 real ones
Yes. That is done using your email client.
>> Is it 200 one use, or 200 at a time (if you have 100 left to use and delete one of your current ones, do you then have 101 left to use) ?
Again, I'd suspect, depends on the company.. looks like a specific set of options, do you have a particular ISP / hosting package in mind?