314 accounts are created at the start of ethereum frontier release and some funds were alloted to them. https://www.etherchain.org/charts/totalAccounts
I'm not sure this is correct. There are 8947 Ethereum presale transactions from 8890 distinct addresses. There are 2 anomalous empty addresses, thought to be a result of a buggy genesis tool, and then there are the Foundation addresses.
So around 9000 accounts at genesis.
I'd suspect the chart you've referenced doesn't include the genesis accounts, but only considers user-created accounts after the start of the chain.
Sources: