When using the Insert Object command to put Prism graphs into other Windows programs, I can't insert more than 16 objects.

When you use the Insert Object command from Excel (or elsewhere) to insert a Prism file as an object, that file remains open within Prism. Repeat 16 times and you have 16 files open in Prism. That's the limit. Prism can't have more than 16 files open at once. Close some of those files using Prism's Close command, and you'll be able to keep inserting more objects.