Output current capability of LDO
Your powersupply must deliver at least 300mA if you are expecting 300mA out of the regulator.

The regulator doesn't generate the output current.  It reduces the voltage and passes current from the power supply.



Low drop out (LDO) regulators are a subclass of linear regulators.  All linear regulators require a voltage difference between input and output (the input must be higher than the output by some amount.)  That is the "drop out."  LDO regulators can operate with a small difference between input and output.

Please differentiate between linear regulators and LDO regulators.   If you mean just any linear regulator, call it a linear regulator.  Only call it an LDO if you really mean "a  regulator with the low dropout characteristic."