PPPoE subinterfaces can be distributed in any way across I/O module ports. For example, you can configure the maximum supported number of PPPoE subinterfaces on one port of an FE-2 I/O module and no PPPoE subinterfaces on the other port.
For information about current system maximums supported for PPPoE interfaces and subinterfaces, see JUNOSe Release Notes, Appendix A, System Maximums.