but when will the limit of bitcoins be mined be reached?
Around 2140, based on estimations.
The circulation limit of the asset will not be just 21 million (value estimated by the system), it will be 21 million minus the bitcoins lost in wallets, which according to studies, is almost 4 million lost coins. This value will probably increase significantly as the year goes on.
Nope, including the lost Bitcoins they will be all 21 million. So, if there was 4 million bitcoins lost forever then plus 17 million supply, in total of 21 million bitcoins will be mined until that year when it will be lastly mined.
Just to give a note, according to coinmarketcap: we have 19,775,137 BTC in circulation already. And with all of the lost bitcoins, if they're totally lost then they're gone forever unless someone is just fooling around trying to pretend they've lost it and then will access it later on.