Technically any amount of BTC2 above 0 is enough. But to have more than a tiny chance, at least 10 BTC2 needed. To averagely get one block reward per week, around 70 BTC2 would be needed right now. If you wanted 10 rewards per week, then 700 BTC2, and so on.
The more coins used for staking, the higher power of staking to find blocks. Consequently, the more regularly staking rewards stakers will receive. I understood what you meant, with 700 Bitcoin 2, staker should split them into ten chunks to maximise staking rewards.