It is recommended to stake 2%-5% of token total market value if a token wants to be listed on Bancor: https://support.bancor.network/hc/en-us/articles/360001240672-How-many-tokens-should-I-stake-in-my-Relay-
But please clarify how many BNT must be purchased?
From the opening paragraph of the knowledge base article that you linked:
Your Relay Token is a new Smart Token with two connectors, each with 50% weight, one holds a balance in your token, the other holds a balance in BNT. You deposit both your token and BNT into your Relay Token.
And from the paragraph you included: It is recommended to stake 2-5% of token total market value...
So it's dependent on total market value of the token being listed.