1
Let's assume that we have N wallet addresses - Addr1, Addr2,.... AddrN, Is there any way to know how many of these addresses are owned by a single entity on the considerations that there's no direct or Indirect transaction links between any of these accounts and all of them are making transactions independently.
3
There is no clear way to do this.
Chain analysis techniques exist that can attempt to identify relations between addresses based on transaction patterns. However, these are not guaranteed to work and often miss relationships or result in false positives.
This "feature" is why it is recommended to use addresses only once, since it is hard to link multiple addresses to a single owner.
