Extreme obesity

At this point it really comes down to how you are built really. And if you keep Your legs strong. I mean I'm a little north of 500 And I still work a fairly physical job. It's not easy. But I'm also over six foot tall. And has always bin a big guy before I put on a bunch of weight.

You will still be mobile past 600 but it will be more limited. More breaks to catch your breath. Some things will be much more difficult or even impossible.

That anew weight goal for you bud^^

I've seen people who were mobile at a level where they could walk consistently and without assistance in the 600-pound range (maybe low 700-pound range as well), and most were pretty much what BigFox described above. Meaning male, tall, solid build, etc.

Yes, but it depends on each person.
I weigh 700 pounds and am not completely immobile, but I need help getting out of bed and off the couch.
Outside of my house I use a wheelchair.

I worked with a lady who was 5'1" and claimed to be over 600 (which I don't doubt) whose only mobility issue was getting both security doors open so she could go through before the lock re-engaged.
She was in better shape than I was despite being 200 pounds heavier.