Monteith is from the Scottish Gaelic Monadh (mountain) + Teith (the River Teith). Monteith Avenue — historic Glasgow boulevard. Lea Michele Monteith — Cory's Glee co-star whose son Ever Leo Monteith was named partly in his honor (born 2020).
Monteith 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 Monteith reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.