Hello Niels
By default, BGP will only advertise the best path for a particular destination. However, it is possible to configure BGP to load balance traffic using several methods, both for ECMP and unequal-cost multipath routing. Specifically, you can take a look at these lessons:
You can also find information at the following NetworkLessons notes:
Finally, for more information about fine-tuning your BGP configuration to get a more specific split of load balancing, you can use BGP traffic engineering, a technique that helps you optimize and control traffic flow. More info can be found at this NetworkLessons note on BGP traffic engineering as well as at the following post:
Take a look at this content and if you have further questions on the specific topic, let us know!
I hope this has been helpful!
Laz