* " this many things" is more common. The word "this" demonstrates/emphasises the large number. For example, looking at someone's doll collection: "I didn't know you had this many dolls in your collection".
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"these many things" is possible in the sense of "these things, and there are many of them", but it is a literary-feeling phrase, and not something that people would often use in everyday English.*
"this many things" is more common. The word "this" demonstrates/emphasises the large number. For example, looking at someone's doll collection: "I didn't know you had this many dolls in