and i see many wallet give for 1 user more than 1 address (HD wallet)
Yes. You should use a new address for EVERY transaction.
An address is like an invoice number. It allows you to identify who is paying you, when they paid you, and what they are paying for.
You wouldn't re-use an invoice number, why would you re-use an address? Then if you receive two similar payments you wouldn't be able to determine who they came from or what they were for.
Excuse me for my query here, but if it applies to know whom sent us the payment for what and if an address actually works as an invoice as well as we should use a new address every time for new transaction, don't you think such rule should be applicable in the case of signature campaigns too?
(My query might sound irrelevant, but I am concerned about this rule that one can't change an address during the whole campaign).
so after 10 years if number of users increase can two users get same address ?
No.
The quantity of addresses is MUCH MUCH TOO BIG for humans to ever use them up. The probability of two humans ever having the same properly generated address in all of human history and future is effectively zero.
What about the leaked databases and/or addresses' private keys over few of the websites?
Can't diffrent wallets be allocated such addresses ever? If yes, aren't they prone to the danger of getting hacked the moment some funds arrive there?