I have 20 separate single band NDVI files. I have to import this into GRASS and stack them into a single 20 band image. How can I do that?
You can use r.out.gdal.
See the example on how to export group of image maps as multi-band file:
g.list group i.group group=tm7 subgroup=tm7 input=tm7_10,tm7_20,tm7_30,tm7_40,tm7_50,tm7_60,tm7_70 i.group -l tm7 g.region rast=tm7_10 -p r.out.gdal tm7 output=lsat_multiband.tif gdalinfo lsat_multiband.tif