All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The Istikhaarah is a form of worship. Initially, worshipping is an individual duty. So, no one should do it on behalf of another person unless there is evidence supporting it. On the other hand, there is clear evidence that denies approval for praying on behalf of someone else.
Ibn ‘Abbas narrated that the Prophet said: “No person should pray on behalf of another person or fast on behalf of him.” [An-Nasaa-i]
So, if someone wants to perform the Istikhaarah, he should do it for himself. But one could ask Allah to guide him/her to do what is best. As for repeating the Istikhaarah, the majority of the Hanafi and Maaliki scholars consider that repeating the Istikhaarah seven times is desirable.
In fact, this is asking Allah more emphatically, which is a good thing that pleases Allah - repeating Istikhaarah. The Shaafi’i School of jurisprudence also consider repeating Istikhaarah desirable but did not limit the number of repetitions and this is the most correct opinion.
So, the person who performs the Istikhaarah can repeat it as many times as he wishes.
For more benefit on how to perform Istikhaarah prayer and how to supplicate Allah, please refer to Fataawa 172573, 81434, and 82964.
Allah knows best.