This is what it comes down to , right here.it depends on the provider, that's why it's better to talk to them directly
Supply and demand
Generally Hosting Provider gives 1 Extra IP to buy in order link.
Providing Extra IP's is depends on service provider and their VPS plans.
If you really need additional IP's then you have to request your service provider and give detail information the purpose VPS extra IP's.
I keep hearing "justify why you need the extra IP addresses" but how do providers verify the justification?
Every provider is different and I'm sure they do take into consideration what you think it's the justification for needing more Ip's. Now just because you think your reason justifies getting additional Ip's, doesn't mean they will approve it.
I think this really comes down to what kind of provider you are with and how strict they are with providing additional Ip's.