How many accounts can I open?

As many as you like, on any plan, at the same time. Nothing is charged and nothing is throttled.

There is no cap and no cooldown. Run a 25K and a 150K side by side if you want to see how the same trading behaves against different room.

One rule does span accounts at a real firm: hedging the same instrument across two of them is banned, because it manufactures a payout rather than proving anything. That one is not enforced here, because it needs more than one account in view at once.