The offer does sounds tempting. A free token for every 1 XEM but still we need to think twice or thrice before jumping into this thing. We better know what will happen next to the XEM we are holding and is it worth the risk to get XYM if we buy loads of XEM? What is in XYM that makes stockpiling XEM fof XYM worthy? We should be clear about this things and don't get hyped to the promise of getting another token for every XEM we are holding.
From what I know, the number of your XEM will remain the same and it won't get deducted for the opt-in. Its literally getting the same amount of XYM for every XEMs you have.
Worthy enough, from what I've read and investigated so far, the blockchain platform where the free token will be used are already being utilized by more than 5 legit businesses across CBD, wine companies, IoT, etc.