The measures of the angles of a triangle add to 180.
45 + 45 + 90 = 180
The measures of these three angles do add to 180, ,so there is at least one triangle with these angle measures.
Using AA triangle similarity, any triangle with the same angle measures will be similar.
From the figure you already see two triangles with angles 45-45-90. There is an infinite number of triangles with those angle measures.