Will I have 10 accounts with 10 different balances?
You just have multiple addresses, in one account.
Ok, understood... so, what will happen if I my 10 addresses hold 1000BTC, divided in 100BTC each, and I want to do a tx of 250 to a new address?, will I have to send it from multiple sources? or Ledger will extract randomly money from each address until it matches 250?
Ledger X is just a hardware which manage the private keys. To manage your coins you need to connect it to any software wallet with coin control. The latter will allow you to select manually those UTXOs (each of 100 BTC ) to combine them into 250 + 50 ( 50 will go to change). Or, if don't want manual UTXO selection, this software wallet choose them for you.