Former Texas governor Rick Perry became the first declared Republican candidate to leave the 2016 race, suspending his campaign on Friday. Perry was averaging less than 1% in recent polls and would have again been relegated to a secondary forum in this Wednesday's CNN-hosted Republican debate.
This leaves 16 Republicans in the 2016 field.
Suspending a Campaign vs. Ending It
If you've ever wondered why candidates 'suspend' their campaigns instead of just declaring them over, it mostly boils down to two things. First, in the unlikely event that the world changes, it is easier to revive the campaign. However, the main reason is that it takes a while to wind down a campaign, paying off bills/debts etc. This article from 2012 discusses some of this in a bit more detail.