Thank you for contacting Intel community.
The differences between EPM3512AQI208-10, and EPM3512AQI208-10N are, the letter N stands for Intel FPGA RoHS compliant device which means EPM3512AQI208-10N are RoHS compliant while EPM3512AQI208-10 are not RoHS compliant. They are both have the same function.
However, sorry to tell you that both of the devices have been obsolete for the past few years. It is recommended to use the latest MAX device family for your design.