From an online search I think the maximum contacts you can merge is 5 by default. I see that this default may be changed but the method may be device and Android version specific.
For your damaged .vcf file you might try to edit a copy of the file with a .vcf editing tool found online, or simply explore it by opening it with a spreadsheet. Be aware of .vcf versions – see
Contacts app > top left 3 bar menu > Settings > Export vCard version.
This will tell you the version in use for export, and also the versions acceptable to /e/OS. (I see versions 2.1, 3.0 and 4.0 so if you use a .vcf editor be sure it will save in one of these versions if you plan to import your finished work.