That depends on how old you are. Early trials suggest that a single dose of the H1N1 vaccine will be sufficient to protect adults and children ages 10 and older. That's good news, since health officials initially thought most people would need two doses. Now twice as many people can be vaccinated with the same number of doses purchased by the government, and people won't have to keep track of their vaccination schedules.
Children under 10, however, will need two doses of the new vaccine, 21 days apart. That's in line with current immunization practices for this age group; all children up to age 10 who are getting vaccinated for the first time against seasonal flu also receive two doses. That's because young immune systems cannot mount as strong a response against influenza as more mature ones can, and since youngsters are less likely than adults to have been previously exposed to influenza, they don't benefit from residual immunity against the virus.
This means some children will need four doses of influenza vaccine this year two for seasonal flu and two for H1N1.