Also can someone explain why 50-500 is the best amount for staking ?
I see that many big wallet just stake 50k plus coins in one address. And some articles suggest that more coins generate more weight and therefore shorten the time before reward.
Can someone explain why ?
I look at it like solo mining. Your weight is your mining rig's hash rate. Network weight is like difficulty. Even though it says that your expected reward is in xxx minutes, it can still take longer than that. There is still variance just like mining.
For example, you are solo mining a coin and the calculator says that you should find a block in 3 days. But because of variance, you have a 99.99% chance of finding one in 30 days. Over the long run your average will be one block every three days.