Where can I find the number of UTXOs my receiving address contain ? Is there any website ?
You can see that on blockchain explorers, like blockchair.com. You can be able to see your UTXOs on most wallets too. But to make it easy, use wallets with coin control. Wallets like Electrum and Bluewallet.
If I reuse an address to receive bitcoin will it count as a second UTXO ?
Yes. If you use an address to receive bitcoin two times and you want to spend the coins or part of the coins from first and second UTXO, it will have 2 inputs and the more the fee.