Good question! It's a strange quirk of the English language. In fact, there's a singular noun which is 'hidden' in this sentence - it's the word 'number'. What we're really saying here is 'finding an astonishing number of settlement sites'. That 'number', obviously, is 198. Here's another example. 'Titanic is her favourite movie. She has seen it an incredible 75 times'.

The word astonishing begins with a vowel sound, so to facilitate pronunciation we change "a" to "an" in such cases. Examples: A dog. An orange dog. An elephant. A big elephant. An ugly duck. A beautiful duck. An honest man. A man.

It goes back to the fact that English originally had a plural form of "one", which usually meant "alone; only". For example, in Blickling Homily XV: Spel Be Petrus 7 Paulus: Þonne be þære lare mines lareowes þe þu me befrune, ne magan þær nænige oþre men onfon, butan þa ane þe mid clænum geleafan hie to þæm gegearwiaþ. "Then concerning the lore of my Teacher, about which you question me, no others may receive it save those alone who prepare themselves for it with pure belief." þa ane "those alone" Ane here is the plural of an "one". But eventually English lost most of its grammatical inflexions, including this "-e" resulting in plural "ane" looking exactly the same as singular "an". So the adjective "one" (which modern "an" is merely an unstressed variation of) in English had, and still has to a limited extent, this plural version, despite how strange it seems for "one" to be used in the plural. It is just a reality of the language. Here's an example of plural "an" from Malory's "Le Morte d'Arthur" - "there he dwelled an eight days" (Book XVII Chapter IV) It is still normal in "a few" and in the negative adjective "no" (short for none "not one")". "A few words", "No words" Otherwise - in Modern English - it is usually only found in more informal/conversational English and accompanied by a fairly emphatic adjective (such as "astonishing" in your example) "an unimpressive ten days" "a measly six dollars"

Thank you

It's like 'another 2 weeks' (a time period all as one), so you need 'a', and 'astonishing' starts with a vowel sound..