You are not limited to an exact number. The phone's memory limits you. It can be limited by how many ringtones, screensavers, themes, etc. you have installed on the phone. Each item just takes memory from a shared bucket of memory.
(You can choose any picture under MyPictures to be a Caller ID pic.)
Go to pictures, click on a picture, select memory status, it will tell you how much free memory you have. Also, be sure to take your photos to be used as a ID tag in the lowest resolution possible, you won't be able to tell a difference on the T68i screen from a 640 X480 photo or a 80X60 photo. You should be able to tag everyone you know at the low resolution. Also, your camera has a memory and on large resolution photos I want to keep I just email it to my computer from the camera memory.