There is no limit on the number of rounds you are allowed to fire. However, you are allowed to own a maximum of 200 rounds in any given caliber, and you are not allowed to own any ammunition in a caliber for which you do not have a licensed weapon.
The exceptions to this are sport shooters, who by nature use up large volumes of ammunition, and collectors, some of which collect unusual ammunition without collecting firearms.
In the case of a sport shooter you will need to apply for dedicated sportsman status, which will mean joining an accredited club and taking part in a certain number of events per year. This will allow you to keep more ammunition, and will allow you to license more than the usual four guns. You will still be required to motivate the licensing of each firearm as relevant to your sporting interests.
In the case of a collector, you will have to join a collector's association and have your field of interest approved. You will then be allowed to license any number of firearms or acquire any types of ammunition provided they can be motivated within your defined field of interest. Well, the licensing is still not a given and definitely not straightforward. They're heavily against 'collectors' who are just in it to acquire more guns.