Hello Michael
The BGP command maximum-paths is used to control the maximum number of parallel paths that can be used for load sharing. As stated in the lesson, this command applies to all prefixes that have the same AS path, next-hop, origin code, MED, and IGP metric, and have a different next hop address. This is how BGP achieves ECMP routing.
However, if you want to apply ECMP to only certain prefixes, they way you could go about doing it is by modifying these attributes of specific prefixes so that you can make them eligible or ineleiglbe for ECMP. You can do this by applying route maps to the prefixes you choose. This is kind of a round about way of doing it, and it can quickly get complex. But it is doable.
I hope this has been helpful!
Laz