change the number of logical partitions
 

Is there a command to change the number of logical partitions in a logical volume? Thanks in advance!

You seem to be talking about creating a new logical partition. There are lots of ways to do that but they all boil down to using partitioning tools of which linux has many. Probably the easiest one to use of the GUI type is gparted which is a Partition Magic clone for linux:

http://gparted.sourceforge.net/screenshots.php

Gparted is available on most linux distros and linux livecds. Popular command line partitioning utilities include cfdisk and parted.

No, the logical partition already exists. I understand you can specify the number of physical partitions when you create a new logical partition but wanted to know if there was a command to change the number of physical partitions on an existing logical partition.

I'm not sure what you mean by "physical partitions". My guess is that you are referring to "primary partitions". A standard PC bios can only recognize, at most, 4 partitions on a single hard drive. These are called primary partitions by most people. To get around the 4 partition limit, a hack was invented very early on to make one or more of the primary partitions act as container for "logical partitions" or "logical volumes"; the terms are used interchangeably. The primary partition acting as the container is generally referred to as an "extended partition". An extended partition is capable of containing an number of logical partitions or logical volumes.

Given the above terminology, I believe what you are asking is whether you can create additional logical partitions on an existing extended partition which already contains one or more logical partitions. The answer is it depends. If your current logical volumes take up all the space available on your extended partition, i.e. there is no unallocated space left on your extended partition, then the answer is no. In that case, you would have to shrink one or more of your current logical partitions by resizing which would leave you with some unallocated space on your extended partition.

However, your question is so vague, it's hard to give you meaningful advice. If you are not familiar with partitioning operations and partitioning tools, proceed with extreme caution. If you don't know what you are doing it's very easy to screw up partitioning operations and lose all your data in the process.