Is there a limit how many pictures a Apple Photos library can hold?

There is no documented limit for the number of photos a Photos Library can hold.

Bute Photos is supposed to be able to open Aperture Libraries, and Aperture supported 1000000 photos, so Photos should be able to do the same. I have not tested Photos with so many photos and videos, however. Currently my largest library has 55000 photos. I do not notice any lagging or performance problems.

But that does not mean, that Photos is able to import too many photos at once. There may be a problem, if you try to import two photos with the same filename together, eve of they are in separate folders. Are the photos you are trying to import on a volume, that is compatible with Photos? Is the file system format MacOS Extended (Journaled) or APFS, and not case-sensitive?

Thanks for the clarification I did miss the point of using photos only for cataloging without importing, and was not aware of this option. Agree if you're not using LR for editing it's an expensive and heavyweight solution. Personally I would never go back to Photos for managing a photo of collection of size (lack of flexibility/functionality, hostage to Apple's upgrades/agenda/vision e.g. recent transition from iTunes to Music in Catalina where they removed functionality).

If you're dealing with that many photos and trying to find the upper limit of Photos Library, then it may be the wrong solution. It physically sucks the photos into its library package resulting in a massive repository (depending on format of photos) that is difficult to backup or move, and whatever conversions need to be done when Apple upgrades things. It's not a scalable solution, these library files just keep growing and difficult to migrate to other solutions once down this rabbit hole.

I had this issue last year with multiple photos libraries and after watching multiple painful file copy operations to backup my hard drive, decided to go a different route. I now store my photos directly in folders on an external hard drive which gives me total control and transparency, and use Adobe Lightroom which catalogs the photos in place by reference rather than duplicating into a repository. I was using Lightroom anyway for editing so this made sense for me to eliminate Photos from the process (LR is a $10/mo subscription to use the service). I now only use a throwaway Photos library to transfer photos from my iPhone to my Mac and then export them directly to disk then import to LR. I currently have 70k photos in LR, a lot in RAW format from my Canon.

This may not work for you but there may be other software that does the same thing... just wanted to caution the long term use of Photos for a big collection like this depending the physical size of your libraries and your expected growth trajectory.

The Filesystem is APFS encoded, residing on a 2 TB SSD.

APFS should be fine for the external volume, as long as it is not case-sensitive.

All these files use the original filename but are stored in different folders.

As test, try to import only the original folders at once, then the folders with the duplicate names, but avoid duplicate filenames during the import. Or use some tool, that allows you to make the filenames unique by appending a suffix to the filename.

  It physically ***** the photos into its library package resulting in a massive repository (depending on format of photos) that is difficult to backup or move, 

I'm going to assume that the asterisk replace the word 'puts'...

But for the sake of less experienced users, I think we should clarify. It copies the files into the library package, unless the user chooses otherwise, and can choose otherwise easily, as this OP in this thread is deciding to do. It does not make the library difficult to back up at all, any basic app that does incremental backups can deal with this effortlessly. Also, any library that has the files copied within it is trivial to move: drag and drop. Ones that opt for having the files without the library package have a much more complex issue with moving.

  I now only use a throwaway Photos library to transfer photos from my iPhone to my Mac and then export them directly to disk then import to LR. 

Your life will be a mite more simple if you learn to use the Import module in LR, which can get photos from an iPhone too. Or failing that, check out Image Capture (in the Applications Folder). Either app will remove Photos from your workflow entirely.

 I am not willing to pay a monthly fee for software i won´t use (photoshop) and within Lightroom i just used the catalogueing function. Which is, as far as i can say, the best catalogue solution, but it is also the most expansive.

Well the good news is that even if you stop paying the subscription with Lightroom Classic the catalogue continues o function. You won't be able to edit or use the Maps, but everything else continues to work.

Incidentally, LR is by no means the most expensive. CaptureOne is significantly more expensive to rent (and there's no Photshop), Mylio, depending on the version you choose and the size of your library, can be more expensive too.

There are a variety of apps that don't require a monthly fee and are also very good library apps. CaptureOne has a one-time licence option, as do OnOne Photo Raw, and PhotoSupreme. I'm happy that you're happy with Luminar 4. I would caution you that the Luminar business model is very strong on marketing and promise, but doesn't have the best record on delivery.

P.S. I'm sure I wrote "physically sucks" but maybe "physically *****" is also accurate.

ok you get the idea