I've seen the poster of the presidential election. The Candidates' numbers begin from 2. Why is there no number one candidate?

There is a pure practical reason for that.

Since 2004 we use new standardized letters and numbers in Finland. In the new system the numbers 1 and 7 are very easily mixed up. That is what happens often in the election of the parlament.

There are eight candidates, and to avoid any confusion the presidential candidates bear no number one.

Sources:
http://www03.edu.fi/oppimateriaalit/kirjainuudistus/numerot_ja_muut_mer…

http://www.hs.fi/politiikka/artikkeli/Arvaa+mitk%C3%A4+n%C3%A4ist%C3%A4…