In hot weather, one layer is usually plenty. A onesie, romper or loose dress made from a breathable fabric does the job on its own. Babies do not regulate their temperature the way adults do, so extra layers meant to look cute or feel protective often just trap heat instead. If you want a quick check, feel the back of the neck or the chest rather than the hands or feet, which are often cooler anyway.
The fabric matters as much as the number of layers. Muslin cotton is woven in an open, loose weave, so air can move through it instead of sitting against the skin. That open weave is what makes it breathable, lightweight and quick to dry after a wash or a sweaty afternoon. Our clothes are double layer muslin, two layers of that same open weave sewn together, so you get a little more coverage and drape without losing the airflow that makes a hot day bearable.
A few practical points. Keep the fit loose rather than snug, since tight elastic and tight necklines hold heat in. Wash on a cold, gentle cycle and skip the fabric softener and high heat, both of which work against the light, breathable weave you are relying on. Muslin also softens with every wash and will crease easily, that is normal and not a flaw. Sizes run from newborn to about 4 years, so there is usually room to size up for a looser, cooler fit if a heatwave hits.