No. Line 42 is for exemptions. Exemptions in 2015 were $4000 per person (this year, $4050 per person). So, $8000 on line 42 makes sense.
$12,600 was the standard deduction for MFJ in 2015. That would be on line 40 of the 2015 1040. If your itemized deductions exceeded the standard, your amount would be higher.
$8000 is the most you could have on line 42 for 2 people, so there's nothing about your question which suggests a problem of any kind.
