Depends on the variety you've chosen - some info in the link below

http://www.empirechestnut.com/faqplant.htm

Hi Katarina, if you have the species rather than a cultivar, then they grow to 100ft x 50ft, so need to be about 100ft apart as a minimum, mind you they will take a while to get to this size, Derek.

Thank you very much, Bamboo, it is very useful information, I was not able to find nothing in Slovak language. I have older tree, which I presented here several years ago, it is Slovak variety of sweet chestnut "Bojar" and the new one is Bouche de Bétizac, with larger nuts. I planted them 7 metres apart.
Thank you, Derek, I think these cultivars do not grow so high, but we will see.