I’ve taken two extracts from the map and reproduced them below, initially highlighting the Southeast Asian nations featuring the highest number of living languages used as a first language.
Southeast Asia doesn’t do quite so well when it comes to the spread of native languages though, with just Malay/(Bahasa) Indonesian being spoken in more than a couple of countries.
On reflection I think the first graph is far more revealing in what it says about Southeast Asia, particularly the geography of the region, with the prevalence of a huge number of islands and valleys leading to the development of historically isolated social groups and thus separate languages.
The creation of those pockets of geographic isolation could, in turn, be attributed in a large part to the plate tectonics of the Pacific Ocean’s Ring of Fire pictured below. What a truly amazing part of the world Southeast Asia is.