Foster is from the Old English forster (forester) — also from foster (to nourish, foster-care). DeMille Award acceptance speech.
Foster 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 Foster reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.