1.- I opened a wallet in blockchain, and I opened also an account at localbitcoins, so now I have 2 wallets right?
No. You have one web based wallet (blockchain.info), and one account at a website (localbitcoins). There is a difference.
With a wallet, you maintain control of your private keys. You are responsible for the security of those private keys, and you can create a backup of the wallet that will allow you to access the bitcoins even if the website disappears.
With an account, you have given a donation to the website, and in exchange you have received a promise from the website that they will do with those bitcoins whatever you ask them to do. They have the private keys, and therefore they have full control of the bitcoins. They are responsible for creating backups, and if the website disappears, the bitcoins are gone. Those bitcoins are no longer yours unless/until the website sends them back to you.
Now, accounts are not necessarily a bad thing. There are many situations where an account can add convenience. Just keep in mind that you have given complete control of the bitcoins to someone else that you've never met and know nothing about, and you are trusting them to do with those bitcoins what you ask.
do you recommend to have two wallets or to consolidate all the BTC in just one wallet?
I have a few dozen wallets. You can have as many as you like. If you find it difficult to keep track of multiple wallets and a hassle moving bitcoins between them, then you can have just one. You may find that you prefer different wallets for different purposes.
For example, you might like to use a paper wallet for long term storage of large quantities of bitcoins that you don't expect to need to access for several months. Then you might like to have a wallet such as Bitcoin-Qt or MultiBit installed on your computer at home for bitcoins that you'll spend at websites while you're on the internet at home, and for storage of bitcoins that you'll use during the next few days or months. Finally, you might like to have a web based wallet (such as blockchain.info) or a smartphone based wallet for small amounts of bitcoins that you'd like to have with you while you are traveling or otherwise not at home.
2.- I am aware that I can create a new address of my wallet to received BTC, but what is the porpouse of generating different adress to received BTC?
There are several purposes.
If you give out a different address to each person or company that will be sending you bitcoins, then you can determine who gave you which bitcoins and when. If 2 bitcoins suddenly show up in your wallet, you don't have to wonder where they came from. Just look at the receiving address, see who you gave that address to, and you know where the payment came from.
Using multiple addresses also increases privacy. If you use a single address then everyone who sends you bitcoins will know exactly how many bitcoins you are holding, since they can all look up the funds controlled by that address. If you give a new address to each sender (or better yet, a new address for every transaction), then they won't know what other addresses you have and therefore won't know how much bitcoins you are holding.
Using a new address for every transaction increases security slightly as well. The first time that bitcoins are ever received at an address, they are protected by three levels of cryptography (ECDSA, SHA-256, & RIPEMD-160). As soon as you send bitcoins that were received at an address, any bitcoins still associated with that address are protected only by ECDSA. They lose the protection of SHA-256 and RIPEMD-160. By using a new address for every transaction, you reduce (or eliminate, depending on the wallet you are using) the the chance that any bitcoins are left in an address that has previously received bitcoins that have since been spent.
does each address only works once or many times, or why we can create many adresses.?
As I said, it "works" as many times as you'd like, but you lose security, anonymity, privacy, and usefull information by re-using addresses.
3.- And the last question, bit coins is a currency that the value depends on demand, I mean there is no goverment nor central bank who controls the value of BTC, so what happened if in the future no one uses BTC,
If nobody wants it, then its value is zero.
and that could be the failure of BTC,
Yes. It could be. If nobody wants it. However, given its usefulness, and security, it seems unlikely that nobody will want it. It is important to recognize though that this is still experimental and carries a significant risk.
or what could happen when the 21 million BTC are produced?
There will never be 21 million. There will be less than 20,999,999.9769. I'm not sure I understand the question. There are lots of things that could happen. We aren't likely to see the last fraction of a bitcoin mined for another 120 years, so attempting to predict the future that far out would be foolish.
another currency will come out
I suppose it could if it offered an advantage over bitcoin.
or they will have to keep making more BTc?
That will almost certainly not happen.
Thanks for your answer.
You're welcome.
