My gut tells me the correct answer is "50-or-so candles," since the "50-or-so" is a modifier of candles -- just how many candles are we talking?
Hyphenation is one of the slipperiest parts of usage, and style guides differ. A common rule of thumb is that compound modifiers of nouns are hyphenated, but you typically don't see this stricture given as an absolute dictate (see, for example, The Chicago Manual of Style). Rather, you hyphenate when there would otherwise be ambiguity. For example, a "fast-sailing ship" is not necessarily the same as a "fast sailing ship."
In your example, there's no ambiguity in omitting the hyphens, and so if you choose to leave them out, there's no real problem unless you're writing for a publication whose style guide mandates hyphenated modifiers in every instance.
I think 50-or-so candles is definitely right, but sadly correct hyphenation is falling in disgrace, so most people will probably regard it as wrong, even if it isn't. Therefore, I would suggest completely avoiding that wording. In addition to the question of hyphenation, it is ugly and lazy. "About 50/fifty candles" does the job at least as well, it is shorter, and there are no hyphens.
See this: http://dictionary.cambridge.org/grammar/british-grammar/word-formation/hyphens. This guide explains when hyphens are "usually employed" and explicitly mentions that hyphens are becoming less common nowadays. However, the way I learnt it 100 years ago [♣], these rules are not optional, so I would recommend you always follow them, especially in formal writing (for example, in academical publications).
Also worth checking out: http://dictionary.cambridge.org/grammar/british-grammar/about-nouns/nouns-compound-nouns
[♣] Just kidding. I'm 37. However, the internet sometimes makes me feel very old. I have the feeling that the English language has deteriorated rapidly since the advent of the internet and mobile messaging.