How much data can I store on my 2 TB HD?

If it's truly an HDD (not an SSD) then you will be better off limiting your storage to a max 1.6GB (80% of drive capacity). This is to allow plenty of space for macOS & applications to operate plus the space needed for system & application caches & temp storage - all are variable in size.

Also understand that HDD performance declines as the amount of used space grows; once you reach 50% usage the decline becomes progressively more noticeable. Plus, APFS itself is not kind to HDD performance.

And make sure you regularly make good backups of your data ... it would be a nightmare to lose 1.6GB data.

It all depends on the individual file size, with photos, there are so many variables that without knowing the file sizes a total count can't be calculated. Never the less a 2TB disk has a lot of storage and you can always buy more external USB drives if your needs expand.

You need to always have at least 20GB of free storage space available on a boot drive for the normal operation of macOS. If your workload uses up a lot of temporary storage, then you should allow for even more free storage space. Bad things tend to happen if the free storage space runs out.

The screenshot doesn't indicate what kind of drive it is, only that it is formatted as APFS. However, in 2019 the only 2TB option for an iMac was a 2TB Fusion drive, which is a 2TB hard drive that has a 128GB SSD coupled to it to kick performance up a bit - that's probably what you have.

Time Machine backups work fine for most people, but keep in mind that it's only 1 backup, and it's exclusively Time Machine.

I'd suggest at least 1 more backup drive to back up important data on a regular basis. That can be done via the Finder if it's not inconvenient to do manually. I use Carbon Copy Cloner to automate 2x daily backups of critical data to an external drive. These backups are of the actual data folders & files and as such can be used immediately on any other Mac without having to go through any TM gyrations to recover files.

Most experts recommend what is called a "3-2-1" backup strategy: 3 copies of your data consisting of 2 local backups on different devices (drives) and 1 offsite backup (cloud or drive).