You seem to be mixing up voltage and current.
You will only get up to 300mA from the regulator, it's the limiting factor. The current capability of the supply is that, only a capability, it doesn't produce that current unless a suitable load is placed across it.
However if you actually draw 300mA from the regulator you have to look at the data sheet to see what the power dissipation is. Without a heat sink it is likely that you won't even get this before it overheats. Most regulators have a thermal cut out to prevent damage.