You should not use more IPs than necessary. A volume less than 10K per day per IP is too low. A volume of 1M per day per IP is on the upper end.
For a mailing list of 1M I would use a pool of 4 IPs. If you are an ESP I would create pools for groups of customers. And reserve the remaining IPs for customers that prefer dedicated IPs.
To warmup the reputation on the IPs you need to start with low volume, or split the volume using cold-virtual-mta and forward the remainder to another platform.
Make sure all prerequisites are in place: authentication, feedback loops, reputation monitoring, ...
Maarten
Postmastery
