You can be selective about which ones you keep – you should, at least, keep the most recent backups because if your catalog is corrupt you will want to be able to recover using these. If the most recent backup has issues then you would progress backwards until you get one which isn’t corrupt.
You need which ever backups you might ever need to roll back to. Sometimes corruption is in the catalog for a while before you go to a folder in LR where you see a problem so it doesn’t hurt to have several older backups, but probably not every one.
It depends on how often you do them, but I usually keep all the recent ones, one a month for a year, then a few a year, then maybe only each time I’ve upgraded my catalog to a new LR version in case I want to go back to that with an older version of LR.
A saved catalog backup saves the entire catalog as it is at that time, not just the changes you made in that session. Thus, the most important catalog backup would be the last one saved, as it would include every change made to that saved catalog date. Remember, however, that if you had a work session and closed the catalog without saving it and the next time you open the catalog and it is corrupt the work you did previously, without saving the changed catalog, will not be saved. Saving each time you do a large amount of work is a good work practice. Saving your catalog backup to your Pictures folder may not be the best choice unless you are also saving a copy to an external drive. If the computer fails and the only backup you have is on the computer then you will lose all of your work if the LR catalog is also corrupt at the same time. Having everything backed up on separate drives is a good work flow practice.