You can put all the updates on one SD card. I have a body and two lenses that required updates. I put all three on a card. The update process gives you a choice of updating a body or lens. I chose body and it did it with no trouble. (Remember to turn off the camera after each update completes.) I then chose the lens update choice. I turned out I had downloaded the wrong file for the lenses I had. It said not file was available for the lens I have. I got the correct file (now I have 4 files on the SD card). It picked the one it wanted and did the update. I changed lenses and did the process again. Again, no trouble.
I suspect that if you had every X-series camera and XF and XC lens that's available, you could still put all the files on one card. The camera seems to be pretty smart about what it needs for what is has.