correctly. The more coins, the faster you get the reward. that to receive a reward every day you need about 300-400 coins.
are we talking chances of getting the reward or the speed of getting the reward or maybe both? For example there's a guaranteed 20% weekly stake of ONION if you got 100 coins and wear their signature and post a bit. At least that they guarantee
If it's pure POS then it is "quite guaranteed". There is variance with POS of course but over time your POS rewards should accurately reflect the amount of coins you hold in relation to all coins actively staking.