Ive got a 10ft x 10ft space to put a shed at the end of the veggie patch.

Ive also got about 16 of these left after ripping up the old decking.

So the idea is dig down to solid clay (not far) big dollop of cement and plonk those on it, press down untill level, repeat as nesecary.

Is there a rule of thumb for how close they need to be? If the base is made of 6×2 how close together do they need to be to suppourt it? One at every joist, every other joist and use hangers on the intermediate ones? Do they need supporting mid joist?

The more the better. How many have you got?

Use them as needed, what else are you going to do with them?

Concrete is cheap

Over engineer

Yep, what WCA said.
Could have done with more joist support in my shed so yes more is better!

Use the Deckin Block Calculator
on here , for sheds

Heavy Duty Deckin’ Block

Grid of 16 blocks is going to give you a span of a bit over 1m.

2×6 will cover that