How many isomers does propene have?

Isomers – in simple words, one composition, but a different structure. There is structural isomerism within one class of compounds and interclass. There is also spatial isomerism (relative to the multiple bond).
Propene (alkenes class) has no structural isomers within the class (the chain cannot be changed, the bond cannot be moved), but there is an interclass cyclopropane (cycloalkane)
Now about the spatial
CH2 = CH-CH3

The first carbon has 2 hydrogens, as if they are not rotated in space, nothing will change, and the second has hydrogen (H) and methyl (-CH3). Here they can be swapped. Roughly speaking, (H) ABOVE the second carbon, and (-CH3) UNDER the second carbon is the cis-form. If it is the other way around (H) UNDER the second carbon, and (-CH3) ABOVE the second carbon, this is the trans form.

Considering that propene is initially already in one of the spatial forms, let it be the cis-form, then there are only 2 isomers: trans-form and cyclopropane.