I have read of one catalogue that was for 13 million files. I doubt all the files were on one drive. As I stated since the catalogue itself is a database there is no known limit to it.My setup is similar, multiple hard drives for images and for backup,. external and a separate file server mapped drives.I have my entire file system of all my photographs backed up on multiple external hard drives. On my computer I have a 1TB SSD for program files and the operating system and a 2TB hard drive for the the last 5 years of photographs. How would Lightroom handle that as I switched from drive to drive?
Lightroom Classic does not care where your files are located as long as they are on local or NAS drives. In my case, I have 180,000 image files located on two different 8TB spinning drives. Those drives and the folders/files they contain show up in Lightroom Classic Library Module for navigation just like thery would in the OS. Files and folders that have not been imported will only show up in Lightroom Classic when you are in the "Import" function.
IMO if you open LrC's Library page and it knows all of the external drives it is easier to use it to move folders around instead of the OS. They are all there in front of you.
Info for others. When you plug in a new drive LrC won't recognize it because it has never worked with it. There may be other ways to do this but I just put one file into a folder. Using the OS I move that folder to the external drive and import that folder from that there. Once LrC sees where it comes from it will always remember it.
Also info for others. Using LrC to move files/folders around is not always the best option. If you are moving tens of thousands of files then it better to use the OS. Move by using the root folder. When the OS is done then go to that root folder in the LrC and it will show an exclamation mark! just right click on it and point it to where you moved it. Moving a mass amount of files is not something you do every day. I've only done that about 3 times. Moving/copying files to a new drive is one example.