Like the standard temperature and pressure set by the IUPAC, the standard laboratory condition (SLC) refers to the standard temperature and pressure values in a laboratory. The temperature and pressure is does not have a uniform value for all countries. In the US, the temperature at SLC is 25 degree Celsius and the pressure is one atmosphere; at this temperature and pressure the volume of one mole of a gas is equal to 24.5 liters.
One mole of a gas has 6.023*10^23 particles.
1.23*10^22 atoms of helium are equivalent to 1.23*10^22/6.023*10^23 mole = 0.0204 moles.
At SLC one mole of a gas occupies 24.5 l, 0.0204 mole of a gas occupies 0.0204*24.5 = 0.5 l.