No, you can’t have just one dwarf gourami. Dwarf gouramis are social fish and thrive best in a group.
Dwarf gouramis are naturally timid. If they’re alone, they might stay in hiding all the time. Moreover, a lonely dwarf gourami might become stressed, refuse to eat, fall ill, and die.
You should keep at least four dwarf gouramis together. If you have the space, then six or more dwarf gouramis are preferable. You can also provide them with other tankmates, such as tetras, corys, and plecos. However, if you have limited space, try keeping at least a pair of dwarf gouramis so they can keep each other company.