The BR-1200CD has 10,000 notes/events available in the pattern memory--regardless of the amount of user patterns that have been created. Each time a note is recorded into a User pattern (e.g. a kick drum, a hi hat, or a bass note), it's taking away available memory from the Pattern memory. This is how it's possible to use up the memory with only a few patterns. It's not the number of patterns you have, it's the content within them. Deleting unwanted patterns is one of the ways to free up the memory. Keep in mind though that the patterns are what make up the songs; so if you delete patterns that are used in the songs, then your songs won't have content within them.
You can also consider creating shorter patterns that have reduced content in them. This minimizes the use of the available memory. For example, if you need to create a section in a song that is 16 measures long, it would save memory by creating a 4 measure pattern (not 16 measures) and play that pattern 4 times to cover the 16 measure section.
The following articles will guide you through creating a drum Pattern and then placing drum patterns into Arrangements:
Creating a Drum Pattern
Creating a Rhythm Arrangement