(John 14:2)"In my Father's house are many mansions; if it were no so, I would have told you. I go to prepare a place for you." (KJV)
(John 14:2)"In my Father's house are many rooms. If it were not so, would I have told you that I go to prepare a place for you?" (ESV)
The King James Version has "many mansions" while the ESV has "many rooms." Most modern translations have either rooms or dwelling places. Does saying in a house are many mansions even make sense in modern English?
In 1611 when the King James Version was originally done, the word mansions could actually mean something like rooms. So rooms or dwelling places is actually the best translation. It is worth noting that the Geneva Bible which preceded the King James Version, had dwelling places.
My Father's house refers to heaven and there will be plenty of room for God's people there. When Jesus says that he goes to prepare a place for you, he probably does not mean it in a literal sense. The Lord likely means that through his death and resurrection, he prepares a place for us in heaven.