It is probably a bad translation from the Italian. More usual would be something similar to "How many places (are we setting/laying) at table?", i.e. how many people are expected.
Although linguistically correct, the Italian isn't exactly something for your phrasebook either. The normal phrase for "to set/lay the table" is apparecchiare la tavola, and for the place-settings apparecchiare i posti a tavola. So I'd simply ask Quanti posti a tavola?
Maybe Duo has to use sedia because posto isn't introduced yet.
'How many chairs shall we put at the table' should absolutely be accepted! Idiomatically it works much better in the English
I'm very much aware that the mods don't frequent these boards as well but come on guys, at least get the English square - 'how many chairs do we put at the table' would be accurate, sure, but commonly it's a question about what is to be done, not just what is done habitually, and the translations marked as correct should reflect that as an option