Thanks!  :)

For other newbies : go to https://wallet.bitshares.org/#/advanced/console then type blockchain_list_address_balances and the address just after on the same line.

you need the address .. and can use
blockchain_list_address_balances

Hello everybody, I would like to find how many BTS there is on a specific Bitshares public key or address.

I found nothing on http://www.bitsharesblocks.com/

Thanks for your help.