I would suggest always buying in pairs (meaning dual channel and not two individual 8gb sticks). So if you but 2x8 now, I would suggest going for another 2x8 in the future. Don't forget to check your motherboard specs to see the RAM specs you can add.
But yes provided that your motherboard can accept 16GB sticks you should be able to run 1 or 2 16GB sticks giving you 32GB or 48GB.
Having said that it is a lot easier to get matched RAM, otherwise you may have to manually adjust bios to the lowest performance numbers. there is a small chance that mismatched RAM may not play nicely, although in 20 years i have never had a problem.