How many solar panels are needed to produce 300 units (kWh) per month?

Depends on location, orientation, shading and other factors.

Where I live, a nice south facing solar array on my roof with minimal shading produces 1780kWh/KW of panels per year. Your asking about 300*12 =3600kWh per year.

If we were neighbors and you had a nice south facing roof with no shade you’d need 2.1KW of solar roughly. This looks like 6~ solar panels.

However if you were my uncle who lives in Salt Lake City Utah, and you have lots of snow, and your roof is east/west and you really like all the trees you planted, you’d only generate 1100kWh/KW of solar, meaning you would need 3.3KW roughly. Or 11 panels.