Should I worry if I see one cockroach?
It would be extremely unusual to have only one roach in a house. Where did the roach come from? Most likely, the roach had a parent and a bunch of siblings, so I would advise anyone that if they see one cockroach, they are probably just seeing the tip of the iceberg. Roaches are expert hiders and skilled at remaining unseen, so it is not uncommon to have roaches without seeing them.