As apparent from herd output, there are four execution candidates:

%  herd7 -cat empty.cat R.litmus 
Test R Allowed
States 4
1:X3=0; [y]=1;
1:X3=0; [y]=2;
1:X3=1; [y]=1;
1:X3=1; [y]=2;
Ok
Witnesses
Positive: 1 Negative: 3
Condition exists ([y]=2 /\ 1:X3=0)
Observation R Sometimes 1 3
Time R 0.01
Hash=78c40b6f8d16b9e5ea74321e4bfc1c40

Both the count of witnesses Positive: 1 Negative: 3 and the observation summary Observation R Sometimes 1 3 suffices to compute the number of executions.

It is also possible to display the executions:

%  herd7 -conf show.cfg -cat empty.cat -show all -gv R.litmus

The command above produces four images, one per execution.

      

We now clearly see that the four executions results from two possible choices of ⁠rf for the read of x “d” combined with two possible choices for ordering the writes to y “b” and “c”.