The Maxima plugin maps the int function to Maxima's integrate, which tries to find a symbolic solution.
For numerical approximations use the quadpack functions (see handbook under Insert> Maxima> Help)
There is also a macro for double integrals, yet I found this to have a cumbersome user interface, therefore I didn't even try it.
quadpack.sm (11.87 KiB) downloaded 1027 time(s).
Technische Mechanik mit SMath Studio: https://link.springer.com/book/10.1007/978-3-658-50592-9