Storage expansion via MicroSD card is not possible on the iPhones. You only get what your native storage has. Typically, the iPhone lineup starts with 64GB internal storage and goes up to 512GB. This year, Apple ditched the 64GB version for the Pro models and made 128GB a standard. Now, a tipster suggests we might see a step further and get 1TB internal storage on the iPhone 13 models.
If you are wondering, the 512GB storage option was unveiled by Apple in 2018. The iPhone XS was the first one to feature 512GB internal storage. Currently, we get to see 64GB, 128GB, 256GB for the vanilla iPhone 12 Mini, and iPhone 12. Whereas, the iPhone 12 Pro lineup gets 128GB, 256GB, and 512GB versions. This year they ditched the 64GB for the iPhone 12 Pro versions.