Number of independent KVL and KCL
I know that the number of KTL and KCL is related to the number of nodes \$N\$. For KCL there are \$N-1\$ equations while for the KTL there are \$L-(N-1)=L-N+1\$ equations.


You should add one word:
For KCL there are \$N-1\$ independent equations while for the KVL there are \$L-(N-1)=L-N+1\$ independent equations.


  However I didn't understand how calculate L. Is it the number of elements in the circuit? Is it the number of branches of the circuit?


Yes, L is the number of elements in the circuit.


  For the KCL is straightforward because there are only \$2\$ nodes [...].


This is incorrect. A node doesn't need to be a black dot...