How sound is the opinion that states the necessity of a separate expiation for each gathering when repeating an oath about the same thing in different gatherings, and what should be done when unaware of the number of these gatherings?

The scholarly opinions regarding oaths and vows are varied. The preponderant view is that if an oath is repeated concerning a single matter, then only one expiation is due. This is the position of Imam Malik and Imam Ahmad. However, if one offers expiation for each oath, it is more cautious and more scrupulous. In cases where the number of sessions is unknown, you should err on the side of caution and offer expiations that you predominantly believe are not less than the number of oaths.