how much token do i need to buy for a better staking
The staking rewards is 10% per year. So it does not matter what amount you have, it will stake regardless.
For example, 750 Priv brings in about 0.1 - 0.3 Priv rewards each day. Ultimately you are going to receive 10%.
Its better to buy as much as you can.
10% per year for how many years? I see that max supply is fixed so it cannot be an inflationary coin, stakes must reduce and finally finish at some point.