You cannot buy less than 100 Connects when you start.
Upwork currently requires new freelancers to do one of two things before being able to submit a proposal for a job: subscribe to the Freelancer Plus membership plan for $20 USD a month, or purchase 100 Connects for $15 USD. After you fulfill this requirement you will also receive 50 free Connects to further help you get started on Upwork, as well as the free Connects tied to your membership for any subsequent months.