Yes there is a limit of 4 primary partitions PER hard drive, so you could have 24 total on your new server. You should also note that you pay a pretty severe performance penalty using extended partitions on most drives and that enabling extended partitions requires one primary partition, so you will only have 3 left (per drive).
If you want a truly flexible, redundant system, I would recommend creating a minimum number of primary partitions, using RAID10 to provide both redundancy (through mirroring) and concatenation, and then using the LVM to allow Linux to see the result as one big volume. Note that there is a definite performance penality for each layer of software abstraction you add, so at a minimum, find a hardware RAID controller.
Now for dealing with Oracle ... Oracle has the capability of dealing with its storage space in a wide variety of ways. You can install Oracle on a server built as I've described above, you can use Veritas to manage your disks, or you can allow Oracle to access the drives as raw devices (you only partition a couple for your operating system's use and it doesn't even recognize the other drives). Choosing the right configuration for an Oracle database really involves understanding how the database will be used (as well as redundancy and backup strategies). If you're new to Oracle, it would be wise to find a seasoned mentor.