My LDL is 48 since last 2 test and HDL is 48 and 56 in last 2 test, total 105 and 118 last two test, what is correct dose of simvastatin?

No "best" dose. Your numbers are excellent. Unless the Simvastatin is causing side effects, i would continue your current dose. If you lower the dose, your numbers won't be as good. There is no "too low" number for ldl("bad cholesterol") or "too high" for HDL ("good cholesterol"). Studies have suggested that the lower the ldl, the better! you're awfully young - why are you on Simvastatin at all?
Can't know. Simvastatin, while indicated to raise HDL cholesterol should not be used to increase HDL . Hdl of 48 and 56 are normal levels. Simvastatin is used to lower ldl, non HDL cholesterol, and apo b or LDL particle number in patients who need lowering based on their cardiovascular risk profile. See your prescribing physician and they can properly address this issue.