Somebody asked me about Bitcoin and I explained to him many many computers are connected to the Blockchain to keep records of all the accounts (or to keep addresses of the bitcoin and owner?).

The blockchain is the data structure they collectively maintain. It is not an "entity" that can be connected to. The nodes are just connected to each other, and tell each other about updates they learn about.

It's hard to count how many nodes there actually are. There are sites like https://bitnodes.io that keep track of reachable nodes. However, there are many nodes on the network that only connect out to others but can't be connected to; probably in the 100000s.

A further complication is that not all of these nodes are equally relevant. It's easy to spin up a few hundred nodes on AWS, that nobody uses for anything, but do make it look like there are lots of independent users. Worse, there are also a lot of spy nodes on the network that don't do any work, except trying to monitor which nodes are the origin of particular transactions.

In the end, if you run a node yourself, it doesn't matter. It validates everything the network tells it, so if you do that, the part that matters is: 1 node, your own.