In Luke 14, Jesus said someone who wants to build what should first sit down and count the cost?

Answer: A tower

A tower builder first counts the cost; otherwise people will mock, "This man could not finish what he started."

“Which of you, wishing to build a tower, does not first sit down and count the cost to see if he has the resources to complete it? Otherwise, if he lays the foundation and is unable to finish the work, everyone who sees it will ridicule him, saying, ‘This man could not finish what he started to build.’”— Luke 14:28-30 (BSB)