It will support only one file in SD Garmin folder named gmapsupp.img with a maximum size of 4GiB - 1 byte. Using PC loaded data (most modern Topos aren't available for PC install) mapping data from several areas can be combined. You can also use 3rd party tools such as GMapTool to combine 2 or more map *.img files into one, again observing the max. img file size and without exceeding the device's overall limit of map tiles, 2025 individual segments for your eTrex Vista Hcx.

Some additional facts that may be helpful: Garmin states that the largest card supported by your device is 4GB (note that GB is gigabyte and decimal, factor of 1000) whereas the file size limit is 4 GiB (gibibyte is binary, factor of 1024). Probably an 8GB card will actually work in your device but if it doesn't simply partition the card as 4GiB in FAT32 and leave the remaining space unallocated. Windows still reports file and drive sizes as "kB", "MB" and "GB" but in the original binary meaning of the prefix terms, it's really reporting GiB for example.