Question about Lightroom Classic and hard drive space

Just to give you some sizes (you got lots of great answers already), I have about 100k images in my main catalog. I store only the last few months on my internal hard disk and offload my older images to a NAS that uses RAID and is regularly backed up automatically. My catalog file on my internal SSD hard disk is 2 GB large. The previews (I don't use 1:1 previews so these are mostly standard size) are 137 GB large. I don't use smart previews. For 100k images this is probably as small as you can get so count on the catalog and previews taking around 150 GB on your internal. If that is too much (Apple is almost criminal for shipping their base models with only 250 GB SSDs), the best solution is getting yourself a small USB-c SSD external disk (they are absolutely tiny and cheap nowadays even for 1TB), plug it in and tape it to your mac with double-sided tape (so you can forget about it) and put your catalog and previews on it. These are much more affordable than getting a new Mac with a better internal hard disk in it. You can also, as already mentioned, put the catalog and previews on a normal external disk but most, if they are spinning drives, are pretty slow. This is not a problem for the images themselves but catalog access speed is a factor in how fast Lightroom feels and it is best to have it on a SSD drive.

One thing to consider is also doing some other clean up to create more space for the classic catalog. A major thing is to set the built-in Photos app (if you use it) to offload images to iCloud (if you have that). That is done in its preferences ->optimize Mac storage. It will automatically offload any iCloud images to the cloud. The same can be done to Music and the TV app. Another way to do this much easier is to click on the Apple menu, About this Mac-> storage -> Manage and enable "Optimize storage" and "Empty trash automatically". Do not enable "Store in iCloud" in general. You can also hit "review files" and see if there are any really large files on your hard disk that you don't need such as installer dmg files in downloads and such but be careful with that to not throw away files you still need.

No, having the image files online or offline would make no difference to the amount of disk space being used. Care to explain more what the issue is? If you're concerened about space being used up on your internal system drive, bear in mind that the Lightroom catalog, and its associated previews cache(s) will increase in physical size as you add more images to the catalog. So if the catalog is stored on the internal drive, there will be a gradual increase over time in the amount of space taken up on that drive, as you add more images to the catalog (even though the images themselves are on external hard drives).