BitSimple can do 20+ BTC but you would need to fund by bankwire. I don't think you are going to find anyone is going to let you purchase that much using a credit card for the obvious fraud reasons.
While credit cards are convenient for buying things, credit card purchases are not designed for transacting in cash.
Don't know of anyone allowing large debit card based exchange for BTC either.
Credit cards charge merchants a substantial % fee for large transactions.
Credit cards also allow for chargebacks. Once bitcoins are paid, there is no such thing as chargeback.
So if it's discovered that the CC was a stolen one, and chargeback of a $20K transaction occurs,
the Bitcoin seller is on the hook and can't reverse payment to the thief.
As a result, there have to be lots of extra fees to cover the additional risks and costs for the merchant.
If you want to buy significant amounts of BTC in the long run, you will want to pay by some other method,
such as by bank deposit, ETF, check, or wire
