[sciencelogic]

In general, there mustn't be any problem If we multiply the listed precursor chemicals of a reaction for achieving more yield, right?

[MNIO]

what do you mean?

let's use this for an example
  1 C7H6O3 + 1 C4H6O3 ---> 1 C9H8O4 + 1 CH3CO2H

from the stoichiometry, we can see that
  138g salicylic acid + 102g acetic anhydride ---> 180g aspirin + 60g acetic acid

so the theoretical yield of aspirin if we react
    salicylic acid       acetic anhydride         theoretical yield aspirin
         138g                    102g                        180g
         276g                    102g                        180g
         138g                    204g                        180g
         276g                    204g                        360g

Those are theoretical yields.  Actual yield (amount you actually produce and recover)
will be less.  The % yield (actual / theoretical x 100%) varies as you vary process
conditions.  Things like reactor design (shape, mixing, residence time, amounts,
concentrations, temp, pressure, etc) are all important factors to consider.  Things like
separation/purification/recovery processes all have an impact on actual yield.  I'll go out
on a limb here and say for "any particular fixed process", there is an optimum amount of
reactants that maximize actual and % yields.  Meaning, I would hesitate to simply
double amounts of reactants and count on doubling the product yield without properly
"scaling up" the process. 

So.. IF you mean
  (1) in general chemistry class, can I double the amounts of reactants to double products?
      and the answer is yes, if you are careful to consider limiting reagents
  (2) in real life production, can I double amounts of reactants and expect to double
       amounts of final products?  NO.  Not without slight to major modifications of your process.

[sciencelogic]

what do you mean?

let's use this for an example
  1 C7H6O3 + 1 C4H6O3 ---> 1 C9H8O4 + 1 CH3CO2H

from the stoichiometry, we can see that
  138g salicylic acid + 102g acetic anhydride ---> 180g aspirin + 60g acetic acid

so the theoretical yield of aspirin if we react
    salicylic acid       acetic anhydride         theoretical yield aspirin
         138g                    102g                        180g
         276g                    102g                        180g
         138g                    204g                        180g
         276g                    204g                        360g

Those are theoretical yields.  Actual yield (amount you actually produce and recover)
will be less.  The % yield (actual / theoretical x 100%) varies as you vary process
conditions.  Things like reactor design (shape, mixing, residence time, amounts,
concentrations, temp, pressure, etc) are all important factors to consider.  Things like
separation/purification/recovery processes all have an impact on actual yield.  I'll go out
on a limb here and say for "any particular fixed process", there is an optimum amount of
reactants that maximize actual and % yields.  Meaning, I would hesitate to simply
double amounts of reactants and count on doubling the product yield without properly
"scaling up" the process. 

So.. IF you mean
  (1) in general chemistry class, can I double the amounts of reactants to double products?
      and the answer is yes, if you are careful to consider limiting reagents
  (2) in real life production, can I double amounts of reactants and expect to double
       amounts of final products?  NO.  Not without slight to major modifications of your process.

Thanks,
that's exactly what i'm looking for. sorry that my question is really naive. I'm a newbie in chemistry.
so as you said, these are all correct. right?

        salicylic acid       acetic anhydride         theoretical yield aspirin
1 x         138g                    102g                        180g
2 x         276g                    204g                        360g
5 x         690g                    510g                        900g
10x       1380g                  1020g                      1800g

[MNIO]

yes those are all correct.. "theoretical yields".  make sure you watch the stoichiometric ratios.  And again, your actual yields will be less than theoretical yields and approximately scaleable.. usually.