More Email Account cost more battery? I have around 15 email accounts, if i setup all that's 15 emails in iPhone with IMAP method.

Will it cost more battery than i just only setup 1 email account?
Yes, it will take more charge than setting up just one account, but that amount will be very negligible, about 1 to 2 minutes a day. On the other hand, it might have a not-so-negligible impact on your data usage.