With current inflation 2500 navcoins are needed to have two stakes per week.
Inflation can vary according to the total amount of staking coins, and surely the stakes will not be exactly 2 per week. Some weeks you will have 4 stakes maybe, other weeks 1 or none, but on average 2 stakes per week.
You can try it yourself here: https://static-rewards.nav.community/
P.s. 1 stake = 2 Navcoin.