There is no limit on the number of times you can download the product, however there may be a limit to the number of times you can re-install the product and for the number of simultaneous installations.
For most Magix products you can have a program activated on 2 computers, if you are trying to install on a new computer then you can de-activate one of the previous installation by going into your Magix account at magix.com - login there, Account icon is top right of the home page, with the email address and password used to register the product and select My Products from there you can deactivate the required installation.
If you have reached the re-install limit then you need to contact magix by email to infoservice@magix.net