RSASSA-PSS-SIGN modBits size In RFC 8017 modBits is the length in bits of the RSA modulus n
I am using 2048 RSA key
modBits = 2048?
modBits = 2048 % n?
lets say n = 10 decimal = 1010 binary
then modBits = 4?
in my 2048 RSA key DER file n uses 257 bytes
then modBits = 257 *8 ?
which modBits = ... is true?
$\text{modBits}$ is the length of RSA modulus $n$ for example if you have 17 as RSA modulus than it has $\texttt{10001}$ as base two representation and has $\text{modBits} = 5$
If you generate a modulus $n$ which is 2048-bit than the $\text{modBits} = 2048$. Keep in mind that, we say a number $n$ is 2048-bit when it is between $2^{2047} \leq n \leq 2^{2048}-1$



  lets say n = 10 decimal = 1010 binary
  then modBits = 4?


Yes 4.


  in my 2048 RSA key PEM file n uses 257 bytes
  then modBits = 257 *8 ? 


The leading 00 is coming from ASN.1 encoding of integers. Therefore you have 256-byte key. 


  which one is the value of modBits?


8*256