Aggregating multiple transactions for the ₹ 10,000 threshold — the threshold is per single transaction.
Deducting only on the excess over ₹ 10,000 — TDS is on the full winnings once the threshold is crossed.
Applying 20% rate — section 194BB rate is 30%.
Bare-Act text Section 194BB · Income-tax Act, 1961 · click to expand
Section 194BB - Any person, being a bookmaker or a person to whom a licence has been granted by the Government under any law for the time being in force for horse racing in any race course or for arranging for wagering or betting in any race course, who is responsible for paying to any person any income by way of winnings from any horse race an amount exceeding ten thousand rupees shall, at the time of payment thereof, deduct income-tax thereon at the rates in force.