It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Whoever uses stones should use an odd number. Whoever does this has done well, and whoever does not, there is no harm (on him)."
(b) If this narration is authentic, then the odd number applies after three. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 506