Yahoo's answer is either "Tough, we're not going to tell you"
https://help.yahoo.com/kb/SLN3383.html
or "100"
https://help.yahoo.com/kb/SLN17518.html
depending upon where you look on their website.
Remember that if someone has genuinely managed to hack into your account, changing your password won't necessarily help. If they've harvested your contacts list they can continue sending mail to all of those people, from a completely different server, still making it look as if it's come from your Yahoo account.
Perhaps it's time to get yourself a
decent email service (which definitely rules out both Yahoo and Hotmail/Outlook, which many people regard as totally appalling services).
Many ABers, including myself, use the excellent GMX. (You needn't worry about losing any mail sent to your Yahoo address. You can configure GMX's 'mail collector' facility to forward it all to your new account):
http://www.gmx.co.uk/