How much BRX do you need to get your wallet/address to stake 10 BRK every 10 minutes?
Is this based on the BRK balance of the wallet, how much BRX you need?
However much you need now, will change later. Right now there's not enough people staking yet, input sizes are not optimal, and there's a large portion of BRX not yet distributed (what's on ICO offer at the moment).
Also, why is the guy from the screenshot getting 10 BRK every 10 minutes, is this because he is one of the few who is staking BRK/BRX at the moment and he is the one finding every block? Or is it because he has quite big wallet sizes?
A combination of factors. Not enough people staking yet, difficulty not adjusted sufficiently yet, most people don't have their inputs optimized yet, and he has a large enough stack, properly split for better staking performance.
It'll be at least a couple of months before we have a stable difficulty for staking. Even if the range was settled right now, it will change significantly again as soon as the BRX ICO completes and those coins mature and join the staking arena.