Memorystore hashmap pages and pages in general

i already read some documentation and posts on memory store hashmap pages (what im using) and ordered data store pages, but there are some areas that aren’t very clear.

  1. how many key-value pairs can be stored in one page?
  2. why are my items being stored on random pages (1, 13, 18, 20) when i put them in the hash map?
  3. how many api calls does getting pages count as, and are they counted when you read the items on the page or when you get the pages?