Each special function unit/block has 8 occupied points, except for some special function devices. These 8 points do not occupy 8 points of the actual I/O numbers at the connected positions. Subtract these points from the maximum number of control points of the programmable controller and use the remaining points for the input/output extension units/blocks. Also, check MELSEC-F Series Model selection tool.

For details, refer to the User's Manual (Hardware Edition) for each series.
Example) When the FX3U series is used
Refer to Sections 6.3.1 and 6.8 in the FX3U Series Programmable Controllers User's Manual [Hardware Edition] (Manual number: JY997D16501).