Praise be to Allah.
If someone is unsure about the stoning, and whether he threw six or seven pebbles, he should proceed on the basis of the lower number and throw one more pebble, so that he can be certain about the act of worship.
This applies if uncertainty arises whilst doing the act of worship. If uncertainty arises after finishing the stoning and leaving, then it does not have any impact, because uncertainty that arises after finishing an act of worship does not affect it.
Based on that, if your uncertainty arose after you had left the place of stoning, then you do not have to do anything.
But if the uncertainty arose whilst you were still stoning the Jamrah, or at the end before you left, then you should have thrown another pebble. If you did not do that, then to be on the safe side you should give a mudd of foodstuff, to be given to a poor or needy person in Makkah. The mudd is one quarter of a saa‘, which is equivalent to approximately 750 g, and it should be rice or a similar staple food. If you give a complete meal, that will be sufficient. You can appoint someone to do this on your behalf in Makkah.
What appears to be the case is that if you do not do anything of this nature, there is no blame on you and it does not affect the Hajj of the one who appointed you to do it. That is because there is nothing in the teachings of Islam to specify what is to be done if one pebble is omitted from the stoning of the Jamrah. Rather this is the ijtihad of some of the fuqahaa’. It was narrated from Ahmad (may Allah have mercy on him) that no penalty is due with regard to one or two pebbles. See: al-Mughni (3/257).
Ibn Abi Shaybah narrated in al-Musannaf (4/280) from Ibn ‘Umar that he said: I do not care whether I stone the Jamrah with six or seven pebbles. It was narrated that Taawoos said concerning the one who throws six pebbles: Let him give something in charity. And it was narrated that Ibn Abi Nujayh said: He does not have to do anything.
An-Nawawi (may Allah have mercy on him) said: Concerning one who omits one or two pebbles: we have stated that the more correct view according to our madhhab is that a mudd (should be given in charity) for one pebble (that is omitted), and for two pebbles, two mudds should be given, and for three pebbles, a compensatory sacrifice should be offered. This is the view of Abu Thawr. Ibn al-Mundhir said: Ahmad and Ishaaq said: Nothing is required of him with regard to one pebble. Mujaahid said: Nothing is required of him with regard to one or two pebbles. ‘Ataa’ said: The one who throws six pebbles should give a date or a morsel of food (in charity). Al-Hakam, Hammaad, al-Awzaa‘i, Maalik and al-Maajishoon said: He must offer a compensatory sacrifice for one pebble. ‘Ataa’ said concerning one who omits to throw one pebble: If he is well off, he should offer a compensatory sacrifice, otherwise he should fast for three days.
End quote from al-Majmoo‘ (8/270).
So the matter is easy, praise be to Allah.
And Allah knows best.