This number gets used a lot, but it's grossly misleading. The 30,000 "denominations" number is reached by counting various rites, autocephelous communions, jurisdictions, and national level organizations all as separate "denominations". In other words, Russian, Greek, Serbian, Ukrainian, (etc) Orthodox are all treated as separate denominations, but they are all actually one Church, Eastern Orthodoxy. Latin Rite, Maronite, Byzantine, (etc) Catholics are all treated as separate denominations, but they are all actually one Church, [Roman] Catholic. The various autonomous provinces of the Anglican Communion (e.g. the Church of England, the Church of Ireland, the Anglican Church of Canada, the Anglican Church of Australia (etc) are all still Anglican, and one ecclesiastical body. Likewise, many nation-level organizations are technically distinct denominations, such as the Evangelical Lutheran Church in America, the Evangelical Lutheran Church in Canada, the Church of Sweden, the Church of Denmark, the Evangelical Lutheran Church of Finland, (etc) are national churches or nation-level church organizations that are all in full communion with one another as an international Lutheran fellowship; the same is true of other groups such as Presbyterians, Methodists, etc.
This makes the definition of a "denomination" incredibly fuzzy.
But the idea that there are 30,000 groups all competing to call themselves "True Christians™" is simply not true. Even where there are legitimate differences, say between Catholics and Orthodox, with both Catholics and Orthodox laying claim to being the one true historic and catholic Church with the other having fallen out of communion, there is still a mutual recognition of the other as being Christian, their members as being faithful, believing Christians. For the most part relations between mainstream Christian bodies is friendly, fraternal, and warm--we recognize the fact that we are all baptized Christians, confessing the same Creed, we call one another brother and sister, and we don't doubt the faith and faithfulness of those who are not part of our group. Now step back a couple or few hundred years ago and things would have been quite different, but today the general move is increased cooperation and relationship.
-CryptoLutheran