As soon as the first bit is uploaded in the link, it starts propogating, The acknowledgement for the frame is sent only once the last bit of the frame is received. So, the Round trip time is independent of the first frame we’re sending,
Now, after the last bit of first frame is uploaded in the link, we stil have 50 msec left, We can still send 50/2 packets in the link, but the last frame sent will act as the first frame of the next window. Hence, effectively a total of 25 frames can be in one window including the first one.
For better clarification, consider the same parameters as above, but condier RTT = 4ms,
Now the diagram will look something like this