[linux-lvm] 15 snapshot limit?

With a 768 GB logical volume in a 2235 GB volume group, and 4 GB of memory, I try to set up lots of snapshots (of 45 or 50 GB each) and can't get past 15. Sometimes I'll succeed in generating more than 15 COWs in /dev/mapper, but lvdisplay never shows more than 15 snapshots in the "LV snapshot status source of" listing under the parent volume. I searched and found 15 snapshots mentioned in a couple places like wiki.samba.org/index.php/Shadow_Copies_with_Snapshots - but nothing really authoritative.

Is there really such a limit? Or is it a memory requirement - I seem to remember so many MB memory per GB of storage, but can't find that either. Versions are (variants of) lvm2-2.02.38, device-mapper-1.02.26, kernel-smp-2.6.20 (64 bit).