How many pictures will 500gb hold

The 500GB Drive is capable of storing up to 142,000 digital photos, or 131,000 digital music files, or 410 downloaded digital movies.

En la misma linea, is it worth getting a 1TB SSD?

If you really hate mechanical drives, then a 1tb ssd is worth it. If you're trying to cut prices, then it's probably not unless you have a niche use for it. Just an addendum; For music and video files, a HDD is just fine, so long as your HDD reads faster than your media plays. (It does).

Sabiendo esto, how many photos can you store on 100gb?

And 100 GB is 100 billion bytes. So the simple math is 100,000,000,000 / 1,000,000 = approximately 100,000 photos.

Puede que te preguntes, what is HDD size?

The amount of storage on a hard disk, measured in gigabytes and terabytes. Hard drive capacity is measured by the amount of data users need to store, which is often much greater than the space applications take up. For example, a single video can take four gigabytes of space. We've Come a Long Way!