There is a limit of 16775134 data pages per fragment in IBM Informix.
Is this also applicable to an index that resides in it's own single fragment?
Documentation does not indicate a limit for a dedicated fragment with index only.
No, there is no 16 million page limitation for a dedicated index fragment. However, the following limitations should be taken into consideration:
1) The dbspace page size
2) Index key size (for a given page size)
3) The ONCONFIG FILLFACTOR parameter
4) Sufficient free space in the dbspace dedicated for index storage
See related URL section for link to these limitations