Bitcoin address generation and UTXO scanning I'm developing bitcoin payment processing application.

I've two question to ask:


In each bip32 address, How much UTXO each address should have to consider generating new address to avoid address re-using? for example If address A used twice should I generate a new address or does it have specified number for address re-use before generating new one?
When new address generated, from my understanding , I've to scan following address for each address derivation path to see if each index have fund or not:


Legacy
Segwit
Change address for Legacy
Change address for Segwit



What's best way to scan addresses without submitting so much request (for each index) to blockchain explorer to scan funds?

Thanks in advance.
How much UTXO each address should have to consider generating new address to avoid address re-using?


Preventing address re-use by itself means that an address should not be used more than once. So unless, someone sends you two outputs to the same address in the same transaction (highly unlikely) you are going to end up with one UTXO per address. Preventing address re-use helps your privacy. For example, say you used an address to receive funds from your customer. If you use the same address again, these customers can actually scan the blockchain to see how much amount you have received from others. Not re-using addresses more than once helps protect your privacy.


  When new address generated, from my understanding , I've to scan following address for each address derivation path to see if each index have fund or not?


Depends on what address format are you using for your payment gateway. If you have always generated bech32 addresses, you need not scan the blockchain for P2PKH addresses to see the amount received from your customers. You only need to scan the blockchain for the addresses that you have generated (receiving or change).