Tatjana is the Slavic form of Tatiana — from the Roman family name Tatius. Saint Tatiana of Rome was a 3rd-century deaconess and martyr; her feast day (January 25) is celebrated as Russian Students' Day.
Tatjana is widely used across the Slavic world. The heroine of Pushkin's Eugene Onegin (1833) is Tatyana.
Tatjana does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Tatjana reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.