Is it obligatory to confine oneself to the established prophetic supplications (du'as) and remembrances (adhkar) both inside and outside of prayer, what are the parameters for adding to them, and is it permissible to invoke blessings upon the Prophet using formulations that are not reported or for a specific number?

The questioner has raised three issues: replacing a prescribed dhikr (remembrance of Allah) with another, adding words to a dhikr, and increasing the number of repetitions of a dhikr.

First: Adhkar are of two types: those whose wording is fixed (and may not be replaced except out of necessity), and those whose wording is not fixed (and may be replaced, though it is preferable to use the prescribed wording).

Second: Adding words to a prescribed dhikr is permissible if they are of the same kind, as evidenced by the Companions' additions to the talbiyah (chant during Hajj and Umrah) and what was narrated from the Prophet, peace and blessings be upon him, regarding the praise of one who sneezes.

Third: The answer to this has been provided in previous fatwas.