Sorry to be a bit nit-picky but, unless they have changed things very recently, it is good for any number of transactions, up to the specified limit and within its (short) lifetime.
I would imagine that most people do set the limit to at, or just above, what is needed for the one transaction that they have in mind. However, this isn't essential and you can, say, set it to 100GBP, use it until either the 100GBP or the month is up, and then repeat.
That can be useful if you don't have constant access to a computer but need to order a succession of small items by telephone - and don't want to leave any of them with your "permanent" credit card details.