Both the Pfizer and Moderna vaccines require two doses. The Pfizer vaccine doses are administered three weeks apart, and the Moderna vaccine doses are given 28 days apart. The first dose offers partial protection and the second acts as a booster. Both doses are needed to get the most protection the vaccines have to offer against COVID-19.
The Johnson & Johnson vaccine is a single dose vaccine.
All three COVID vaccines currently available are administered intramuscularly (into the muscle, just like a flu shot).Department of Public Health Vaccine FAQs