You can't just "increase" the amount it can hold. It will hold the amount it is programmed to do so.
Make sure your phone has all of the needed in store software (firmware) updates as well as make sure you do *228 every 3 months. These two things together update and program your phone. Sometimes, Verizon Wireless releases firmware updates that allows you to store more messages.
Also, there isn't necessarily a "number" of messages they put that you can hold..it's more of a size. For example, if you have a lot of pix messages in there...then the storage of messages will be smaller.