Regulator output current
What'll happen if I connect this regulator output to 4 ICs, which will consume about 100mA per each IC.


You'll be asking 400mA from a regulator rated for 300mA.  Depending on the regulator it'll either shut down, or deliver 300mA at whatever voltage it can manage, or try to deliver 400mA and burn up, etc.


  Or what'll happen if I connect this regulator output to 2 ICs, which will consume about 100mA per each IC.


If you haven't run into some thermal pitfall (and power devices can have very deceptive thermal ratings) then the regulator will deliver its rated voltage at 200mA.