I'm going to assume the question states:
How many moles of P4 are needed to produce 10.5 L of P4S3 at STP?
First, use the ideal gas law to find moles of product produced, since the volume is for liters of P4S3.
Solve for n
P = 1 atm
V = 10.5 L of P4S3
R = 0.0821 L⋅atm/K⋅mol
T = 273.15 K
n = (1 atm)(10.5 L)/(0.0821 L⋅atm/K⋅mol)(273.15 K)
n = 0.468 moles P4S3
Then use the balanced equation to convert moles of P4S3 to moles of P4. This is a 1:1 ratio.
0.468 moles P4S3 x (8 moles P4)/(8 moles P4S3) = 0.468 moles P4