In top-tier orchestras you usually 4 oboists, though typically one of them is officially the English Horn player.
Second tier orchestras tend to have only 2 or 3 oboes.
[][][][][][][][][][][][][][][]
It's not so much the 'tier' orchestra, but rather the number of oboes required by a composer in any given piece. A Haydn symphony may require two oboes (or in some of his symphonies: NONE) while a tone-poem by Richard Strauss may require triple WW or more. And so - MOST usually: the number of oboists in a symphony orchestra is dependent upon rep.