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 KiB) downloaded 23 time(s).
Martin Kraska
Pre-configured portable distribution of SMath Studio: https://en.smath.info/wiki/SMath%20with%20Plugins.ashx