[gibybo]

It is definitely a small number. Visa is capable of handling 80 million+ per hour, for example. However, transactions are prioritized by transaction fee so you would have to pay a fee higher than all other transactions to block them out. Currently this would cost you something like $100k per day, but if you started to do this you would rapidly increase the cost of transactions, so the cost to perform this attack would grow much more expensive.