A one-time purchase. All updates to the version you have are free for 12 months from date of purchase; any future upgrade (to a "new" version) are likely to be chargeable.
To add to what @SP. has written, you need to be aware that contracts for monthly subscriptions require the user to give 30 days notice of cancellation. Hence you may well find you are charged for one more month, depending when Magix acknowledge receipt of your email, since the 30 days begins on the date the acknowledgement is sent, I believe.