You have a private blockchain, but you need to understand the public Ethereum blockchain to understand its immutability.
The Ethereum blockchain is a Decentralized Application (Dapp). Unless you are running a private blockchain on just one computer, you don't have an Ethereum blockchain on your computer. You have a copy of it. The Ethereum blockchain is actually copied across thousands and thousands of computers spread around the entire world. If you want to know where the actual blockchain is, it is on 51% (or more) of the computers running Ethereum blockchain nodes, but never on 49% or less, or even on one of the computers.
As a Dapp, the Ethereum blockchain maintains its immutability by spreading itself over too many computers in too many locations to be maliciously altered. You can think of the blockchain as a collective experience. However, it is backed by a lot of mathematical expertise and advanced Dapp research.
Ethereum blockchain immutability is guaranteed because of the number of miners (and soon forgers) keeping track of the blockchain's ledger. The ledger is that data in your chaindata directory. You can change the files in chaindata on one of the nodes, meaning your computer or computers, but the other nodes on the blockchain will know that you have a different version of the blockchain and ignore your node's version. So, the blockchain cannot be altered by modifying the chaindata on one computer.
If your chaindata is altered, it will not sync properly with the actual Ethereum blockchain. In that case, just delete the chaindata completely and re-sync. If your connection to the Internet is good enough and you have a really good SSD in your computer, you should be able to re-sync a full node within a day.
If you are running a private blockchain for a company, you need at least three very secure nodes hosted on three separate servers to maintain a minimum immutability. That is because (as I mentioned above) your blockchain exists on 51% or more of your computers. As you introduce more nodes into your private Ethereum blockchain, you introduce more security and immutability. However, it is unlikely that you will reach the level of security and immutability of the public blockchain.
If you just need a private blockchain for development and testing purposes, then one node on one server will work. Just remember, one node on one server is not in anyway a dapp, so cannot be secure or immutable.