Verify why they need them! You need to justify it to your uplink anyway.
Then it's on a case by case bases. We've had users request the entire block 254 IPs, and we've had user request 5. In each case, they need to justify the needs, most of it was to do with SSL Certificates etc.
Ask them why they need them, the answer could be legit. Maybe they've moving from another hosting company and they need a large block to accommodate their existing SSL certs.
I know when we moved datacenters many years ago, we purchased 50 servers with 32 IPs on each one. We had to justify each, but since they were legit requests, we had no problem.
IPv4 addresses absolutely need to be justified, but at what quantity should you query IPv6 addresses? We get lots of queries asking for ridiculous amounts of IPv6 addresses.
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With IP addresses, there has to be a need for them, there has to be justification, because the end result, is that the justification has to go back to ARIN, who provides IP addresses to datacenters and hosts throughout the world.
Perhaps the client is doing some sort of dedicated hosting or will be running VPS machines from a dedicated server. With our dedicated servers, we can use anywhere from 10-50 IPs one 1 individual server, for VPS machines and some VPS machines need multiple IPs for various reasons. So, you can actually utilize alot of IPs for just 1 dedicated server.
Just make sure that your client provides justification and you supply that to your hosting provider as well. This is a requirement, not just for you and not just for your host, but it comes from ARIN, who provides the IPs to everyone, they must have that for every single IP that is assigned to everyone.