how do i know how much storage to get

Depends on your use. If your use is just web browsing, email, a few photos and some music then you don’t need a ton of storage. If you plan on downloading a large video library, edit video, take thousand of photos, then you’ll need a ton of storage.

Another strategy is to buy the iMac with stock storage and use external storage devices for your personal data. That way when you need more storage you can buy more.

The iMac internal storage cannot be upgraded, you are stuck with what you choose at the time of purchase.

Simple: as much as you can afford.

IMHO, anything less that 500GB will cause to regrets down the road. We are already seeing people with 256GB drive struggling simply do do system upgrades.

Avoid another regret: DO NOT buy the two-port version!!

Get the biggest you can afford but do budget for AppleCare it’s worth having.

In 2019 when I was going to buy a new iMac, I was advised by a patron on this site to get 16g of memory and 1TB of Solid State storage, which I did. The solid state storage cost $600 but I've never regretted it. I know whatever I plan for this Mac, I'll be able to do.