Hi!
There are Multiple ways to do this.
The easiest is probably using the "Mesh>Fill Hole" command, but that will leave you with an N-Gon that you then have to cut up into quads.
You can also use the "Mesh Tools>Append Polygon" tool and draw multiple polygons between the edges.
Or you select edges on opposite sides and use the "Edit Mesh>Bridge" command.
Or you use a combination of those tools. For a hole with 20 vertices I would normals use 4 edges on opposite sides and use a bridge command with 3 Divisions, then bridge between the middle two edges on each side which leaves 4 holes with 4 edges on which I use the Fill hole command.



I hope this helps!