Some doubt with the eigenvalues size of PCA

edit

Assuming the mat is 246*3000,that is, I have 246 data and 3000 feature. Then I use the PCA as below and the result is 246. I think it should be 3000, what wrong with the code？

PCA pca(mat, Mat(), PCA::DATA_AS_ROW);
cout << pca.eigenvalues.rows << endl;