Hi Rodrigo,
This is correct - though surprising. If you look at the max projection for that experiment there is only one visible neuron (you can see this via the SDK, or on the website here: http://observatory.brain-map.org/visualcoding/search/experiment?id=593506466).
Why is there only one cell? I think this is mostly an anomaly. The Cre line, Nr5a1, is sparse in general. The other experiments for Nr5a1 in the same area (VISal) have ~12-30 cells. So this is generally a sparse line. But only having one cell in the field of view does appear to be unusual.