The one file versus many files outcome depends on where you export from, since a single vCard can hold many contacts concatenated, so the routes that produce the combined file and the fix when you end up with many:

From the major sources, one combined file is usually the default: Google Contacts, Export, vCard produces a single vcf with everyone, iCloud contacts selected all then Export vCard gives one file and most phone contact exports produce a single combined vcf. So exporting from these generally gives exactly the one file you want, the format supporting many contacts in one file natively.

When you get many files instead, combining them: the vCard format is plain text and a single file holding many contacts is just their individual vCards concatenated, so combining a folder of separate vcf files into one is a simple concatenation, on Windows copy /b *.vcf combined.vcf in the folder, on Mac or Linux cat *.vcf > combined.vcf, producing one file every contact importer reads as the full set. This is the fix when a source insisted on per contact files.

The Mac specific case, since it commonly produces separate files: dragging multiple selected contacts out of Mac Contacts can produce individual vcf files, so either select all and use File, Export, Export vCard which produces one combined file or concatenate the separate ones with the cat command. The Export vCard menu item is the one combined file route on Mac specifically.

The verification and use: open the combined vcf in a text editor and confirm it contains multiple BEGIN:VCARD blocks, one per contact, the sign it truly holds everyone, then it serves as both a single file backup and a universal import into any contacts system, the one file being far more convenient to store and move than a folder of hundreds. Keep it somewhere safe as a genuine contacts backup, the single portable file the format is good for.