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Currently you can set-up number of IPv6 Proxies when you Add Vultr Account at https://createproxy.com/dashboard/new_vultr/
There you can find field Insert number of IPv6 proxies, that will be created with this account where you can select number of IPv6 proxies that you want to create for VULTR, please note, that you can run aprox 120 IPv6 Proxies on single instance with 0.5 GB / Ram. If you want increase this number, you also need better instance (with more RAM), that can handle required number of IPv6 Proxies.
So If you want to run like 1000 Ipv6 proxies on VULTR single instance, you should probably take instance with 4 GB / Ram etc.
If you need better soluction, I have create IPv6 Proxy Generator, that is part of Merchant Tools, this IPv6 generator may be used for any provider that is offering IPv6 prefix even VULTR VPS/ OVH VPS etc. https://createproxy.com/merchant/server_template