The established Sunnah is to throw exactly seven pebbles at each Jamarah, saying 'Allahu Akbar' with each throw. This is the consistent practice of Hz. Peygamber (sallallahu aleyhi ve sellem) and is considered wajib (farz) by all four schools. Throwing fewer than seven pebbles means the stoning for that Jamarah is incomplete, and the missing pebbles must be thrown to complete it.
Sheikh Ibn Uthaymeen explained that if a haci is uncertain whether they threw six or seven pebbles, they should throw an additional pebble to be safe, as they build on certainty (the lower number). However, deliberately throwing eight or more pebbles is disliked (makruh) because it goes against the prophetic practice and Hz. Peygamber said: 'Use pebbles like these' (referring to small stones) and warned against exaggeration. The extra pebble does not invalidate the stoning but is contrary to the Sunnah.
Regarding the size of pebbles, Sheikh Ibn Baz specified that they should be slightly larger than a chickpea (approximately the size of a date pit or a small bean). Using large rocks, shoes, or other objects is prohibited, as Hz. Peygamber (sallallahu aleyhi ve sellem) specifically warned against extremism (ghuluw) in this context. He said: 'O people, do not kill yourselves. Use pebbles like these,' indicating small stones. The pebbles should be collected from Muzdelife or anywhere along the way — they do not need to be washed, though some hacilar prefer to wash them.