The Arbitration and Conciliation Act, 1996 lets parties fix the number of arbitrators by agreement, provided it is not an even number. Most commercial contracts choose between one and three.
A sole arbitrator suits disputes of modest value, straightforward facts, or where speed and cost matter more than having multiple perspectives on the bench. A three-member tribunal — typically one nominee from each side and a presiding arbitrator agreed between them or the appointing institution — suits high-value, technically dense, or multi-issue disputes where a single decision-maker carries more risk.
Whichever you choose, say so expressly in the clause. Leaving the number unstated invites disagreement about the tribunal's composition at the exact moment a dispute has already arisen and cooperation is hardest to get.