The minimum staking requirement is mentioned in the article, but just in case you missed it, here is what it said!
In the single-shard model, a validator must stake at least 0.2% of the total FTM supply (6,350,000 FTM) of their own tokens. This number, as well as many other network settings, will be subject to change via on-chain voting. They will also change when Sharding is introduced.”
Hope this answers your question!