I have 200 shapefiles (polygons) which I need to union them all into one layer. I need union it in order the overlapping could shows the intensity area. however, i am having problem to union all of the shapefiles. Why this is happening? i only manage to union like 65 files of the shapefiles... the ERROR 000210: Cannot create output... Failed to execute (Union). keeps on popping out... it is stressing me out. i could not do the next step (which is to know the intensity of the overlapping area) if i could not manage to union all of these shapefiles..
It could be that you're exceeding the number of attributes (fields) allowed in a shapefile after union of 65 shapefiles. Or you're exceeding the hard limit for number of bytes in a dbf row. See Geoprocessing considerations for shapefile output for more info about how horrible shapefiles can be for real-world processing.
I'd suggest converting your shapefiles to a file geodatabase, then try union again. (File geodatabase doesn't have a number of fields limitation).
Another idea: merge your shapefiles together (into a file geodatabase), then run union against this merged dataset... yes, you can union a feature class with itself. Try this on a subset of your data and see what you get.