The answer is: not necessarily.
The dosage (milligrams prescribed) can depend on many factors, including--but by no means limited to--the type of depression one has, the response to treatment, the side effects the patient has to any particular dosage, the other conditions the patient has (physical and/or mental), other medications taken in the past (included whether, say, Zoloft itself was taken in the past) and their side effects and/or responses to treatment, past physical and/or mental conditions, other conditions for which the Zoloft is intended to treat (this might affect the dosage), other medications currently taken or chosen to be prescribed in conjunction with [the Zoloft, in this instance], how tolerable the side effects are to the patient as regards to the therapeutic effect, the doctor's judgment of proper ethical prescribing and monitoring.
Also, different people respond differently to medications. Some people might take 200 mg of Zoloft per day with no side effects yet no response to treatment. Others might only tolerate 25 mg per day and not be able to tolerate any higher doses yet have great therapeutic response.
Additionally, Zoloft is prescribed for many conditions--even "off-label" conditions. Zoloft has been prescribed in conjunction with other medications for various types and manifestations--as well as severities--of conditions--and sometimes in conjunction WITH OTHER MEDICATIONS---hence the benefit of being able to adjust the dosage.
Zoloft has been prescribed alone--and in conjunction with other medication--for the mildest forms of depression to dysthymia to seasonal affective disorder to mild/moderate/severe major depression--with and without psychosis; schizophrenia/schizoaffective disorder, sleep disorders/disturbance, lupus, fibromyalgia, migraines, panic disorder, OCD, generalized anxiety disorder, social anxiety disorder.