The specific number of confirmations you should wait for depends on the amount you are receiving, not sending. For example, if you are a merchant selling coffee, you shouldn't wait for 3-6 confirmations to sell each cup of drink, because you assume no one will try to double spend such a small amount. In case your customer wasn't honest and did double spending, you will not lose much and will likely to refuse any payments from that particular customer. If you are selling your house or car, wait for as much confirmations as possible, because in this case risk much higher, I would say 30-100 confirmations is optimal for this, you are not going to risk big amounts.
A.Antonopoulos well explained that in his book "Mastering Bitcoin"
A common misconception about bitcoin transactions is that they must be “confirmed” by waiting 10 minutes for a new block, or up to 60 minutes for a full six confirmations. Although confirmations ensure the transaction has been accepted by the whole network, such a delay is unnecessary for small-value items such as a cup of coffee. A merchant may accept a valid small-value transaction with no confirmations, with no more risk than a credit card payment made without an ID or a signature, as merchants routinely accept today.
the cafe owner, is willing to accept payment for cups of coffee without waiting for confirmation (mining in a block), because the risk of a double-spend on a cup of coffee is low in comparison to the convenience of rapid customer service. This is similar to the practice of coffee shops that accept credit card payments without a signature for amounts below $25, because the risk of a credit-card chargeback is low while the cost of delaying the transaction to obtain a signature is comparatively larger. In contrast, selling a more expensive item for bitcoin runs the risk of a double-spend attack, where the buyer broadcasts a competing transaction that spends the same inputs (UTXO) and cancels the payment to the merchant.
