About the “main network”, there is no centralised place to check (yet), so it’s a hard question:
The IPFS may have an estimate since there are running very popular nodes (Bootstrap nodes, Preload Nodes, Relay Nodes, Gateway Crawler, etc.)
Plus, IPFS nodes can be used to function in a private network (only talking to nodes in the same private network), or in a local network so it’s hard to count them in any way.
There is no “transaction” per se, because IPFS is not a blockchain or a marketplace. It’s a network of nodes storing and exchanging data.
There is a curently-building blockchain on top of IPFS that will provide incentive for that: Filecoin.
There is no “Explorer” as in “Ethereum Explorer” because there is no consensus on a common “list of files available” (or any common “big file” shared by everyone, such as a ledger in the blockchain world).
There could be a search-engine like Google and some others did for the Web, but they are still very WIP or rudimentary.
Exemple: ipfs-search.com , https://www.ipse.io