What RAM to take for Imac 27 2017?

Question

RAM

Hello.
I have a question about the operating system. I bought an Imac with 8 gigs of RAM, I want to add +16. But I want to do it with one plank, so that there is still room for one plank in the future.
Will one bar work correctly, or is it better to take two of 8 each?
In the store where I bought the Mac, they wrote to me:
"In iMac, dual-channel memory and one bar is not accepted. It will work, but not correctly. Therefore, the bars are sold in pairs."