I'm playing the Sims 2: Super Collection game off the Apple Mac Store on my macOS Catalina, Version 10.15.2. I downloaded a bunch of CC recently and it was running fine. But then, I downloaded a lot more, but not insane amount - there's about 1,500 CC files in my "Downloads" folder. But every time I run the game now, it takes a long time loading on the first loading screen with all the squares. And then it just crashes. I took out all the CC files I had downloaded, put them in another folder, and it ran fine. Then, I returned all the CC files and it crashed in the same way again. What can I do to fix this?

Since each batch of files works, you could have a conflict between the two. You will have to continue doing Game_Help:Finding_Problem_Custom_Content^wiki50/50 to find the file issues.
It's also possible you hit the file limit on your Mac

I've read that macs have limit for how many files it can have open at once, you're probably hitting that limit, here's tutorial by lordtyger9 about how to edit the limit but since I know nothing about macs it's better @lordtyger9 takes it from here.

It could also be that you've simply downloaded something that isn't compatible with your game, try adding the files back little by little to see how that goes.

e: ninja'ed by HugeLunatic but I'll leave this 'cause I posted that link.

A couple of people have stated that your problem might be that you have too many files in your Downloads folder, and I think that yes that is the most likely cause of your problems. I have several posts on InSimenator.org on this general topic and I will include them in the order that you should read them.
This explains the symtoms: http://www.insimenator.org/index.ph...c,115344.0.html

How to remove the SIP temporarily that Apple has set up so that you can't change things: http://www.insimenator.org/index.ph...c,115580.0.html

This is how to increase the default numbers so that you can have more cc and your Mac will still work: http://www.insimenator.org/index.php/topic,49183.0.html

{Note I increased mine again because I added more files, I have 9,119 items now so if you plan on having a lot make your Max Open File Number higher than the one I give in my post on InSim}, BTW everything is fine with you increasing the Max Open file number if you have enough RAM. I don't know if this number is the correct number to use if you have over 9000 files like I do, but I use 81920 as my Max Open Files Number. It is what I use and my Mac runs the Sims 2 with the number of files that I have just fine.

Read my posts in order and then just take your time, this is a Technical thing to do, but it is safe and reliable as far as I know if you just take your time, I set most of it up so that you can copy and paste. Just remember to increase the Max Open File Number if you are going to have over 9000 items in your Downloads folder. It is possible to change things once you got every thing set up, but it is a little harder. So giving you a heads up.

Feel free to ask me for help if you have problems.