How to best manage a large Photos library

Store it on an external APFS or MacOS Extended (GUID, case insensitive) local disk. Make regular backups to external disk(s). Split it into smaller libraries. YMMV.

A bit more detail would help. What do you mean by "manage". Are you interested in managing your edits - you need to track what needs doing, what state it's at, what's completed, pending, etc? Or are you interested in categorizing photos to know content - maybe you want hierarchical categories like Places-Europe-France-Paris-Louvre-Mona Lisa

Different apps do different things so the more you tell us the more we can help.

As Zurarczurx asks, are you talking about how to organize the pictures, or how to deal with too many pictures?

You really need to tell us what device you're talking about. Dealing with a Photos Library on an iPhone is different from dealing with it on a Mac. For instance, on a Mac you can just divide the Library into "pictures I look at often" and "pictures I only look at every once in a while."

Let us know:

1. If this is a Mac, do you have more than one Photos Library? 
2. If this is a Mac, where does your Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
3. Are you using iCloud Photos to synchronize your devices? If so
4. …Do you have “Optimize Storage” checked in Photos’ Settings
5. …Do you see all your Mac’s Photos  pictures at iCloud.com?
6. …Have you checked to see if the Apple Account is really really the same on the Mac and at iCloud.com?
7. …What is your storage at iCloud.com? At System Settings>[name]>iCloud>Storage , my Mac says 176 GB of 200 GB used. What does yours say?
8. What is your storage on this Mac? At System Settings>General> Storage, my Mac says 745 GB of 1TBB used. What does yours say?
9. If this is a Mac, what does your Library say at the bottom of the Library view? Like this:                        

If it's an iPhone , check here:

How large is the library?

And are you asking about organizing the media inside the library for an easier access or managing the library package for easier updates and backup?

The size of the library matters primarily when we need to update the photos to a new format after a system upgrade or when we need to move it to new drive or sync it again with iCloud.

When searching for photos or working with smart albums you may notice that Photos is getting a bit slower, when the library is growing larger. I like to use many smart albums, so I try to keep my library smaller than 50000 items.