Usage increase is proportional to the number of marker updates. Every time you update map markers, some computation is required to calculate the optimal position of the markers. The compute is provided by an API endpoint which is used by the library internally.
Usage is billed on a per-million marker updates per month basis.
If you use the same markers on your map multiple times, subsequent compute requests will hit the cache and the usage will not increase. The cache is invalidated after a certain time depending on the subscription tier.