The edge image returned by edge() is a logical image. So to get the length of all the edges, you do
lengthOfEdges = sum(edgeImage);
To get the number of edges:
[labeledImage numberOfEdges] = bwlabel(edgeImage);
To get average edge length (which you didn't ask for but I'm giving you anyway in case you need it), it's
averageEdgeLength = lengthOfEdges / numberOfEdges;
To get the average number of edges you have to define (like Walter asked) what you're averaging over.
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