How can I retrieve the Session ID from command line?

This is certainly not the most efficient way, but you can take a look at the results of tasklist. It will display the name and session# of each process running. If you are logged in locally then look at the ID for the session named "console".

d:\>tasklist

Image Name                     PID Session Name        Session#    Mem Usage
========================= ======== ================ =========== ============
System Idle Process              0 Services                   0         24 K
System                           4 Services                   0      8,580 K
smss.exe                       316 Services                   0      1,500 K
...snip

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Edit:

Query Session will retrieve the session id:

d:\>query session
 SESSIONNAME       USERNAME                 ID  STATE   TYPE        DEVICE
 services                                    0  Disc
>console           janedoe                   1  Active
 rdp-tcp                                 65536  Listen

This combines aspects of Gary’s answer and Oscar’s answer.  As noted by Gary, the output of query session looks something like

 SESSIONNAME       USERNAME                 ID  STATE   TYPE        DEVICE
 services                                    0  Disc
>console           janedoe                   1  Active
 rdp-tcp                                 65536  Listen

or, in my case (using “Switch user”)

 SESSIONNAME       USERNAME                 ID  STATE   TYPE        DEVICE
 services                                    0  Disc
>console           gman                      1  Active
                   otherguy                  2  Disc

(I don’t have RDP set up, so I can’t test with it.)  Presumably the intent of the question is to determine the ID of the currently active session.  I suggest searching the output of query session for what looks like the current session:

 for /f "tokens=2-4" %a in ('query session') do @if "%a"=="%username%" if "%c"=="Active" echo %b

which assigns the 2nd, 3rd, and 4th word (which we hope to be the USERNAME, [SESSION] ID, and STATE) from each line of the output of query session to %a, %b, and%c, respectively, and then reports SESSION ID from the line where USERNAME is me and STATE is Active.

It turns out that query session takes a username parameter, so the above can be simplified to

 for /f "tokens=2-4" %a in ('query session %username%') do @if "%c"=="Active" echo %b

or

for /f "tokens=3-4" %a in ('query session %username%') do @if "%b"=="Active" echo %a

To use this in a script (batch file), you might want to say

set MY_SESSION_ID=unknown
for /f "tokens=3-4" %%a in ('query session %username%') do @if "%%b"=="Active" set MY_SESSION_ID=%%a

using %%a and %%b (instead of %a and %b) because it’s in a script.  You can probably leave out the @ if the entire batch file is under @echo off. The initialization to unknown lets you do error-handling (detecting if there is no line in the output of query session that matches).  Extending the error-handling to recognize and react if there are multiple matching lines in the output of query session is left as an exercise.

for /f "tokens=4 delims= " %%G in ('tasklist /FI "IMAGENAME eq tasklist.exe" /NH') do SET RDP_SESSION=%%G
echo Current RDP Session ID: %RDP_SESSION%