You may well get 50+ bids, but that doesn't mean that 50+ will be worth testing. You can start off by asking yourself some questions and defining your parameters. For example:
What style of English are you looking for? American or British? This is important in light of: 'It's editing for sentence structure and confirming English idiom usage'. Sentence structure and idiom is different. Or you might want a Canadian or South African tone (for example).
What sort of rate are you anticipating paying?
Do you need someone who has an understanding of the source language/culture?
Do you want a particular style guide used? If so, you can dismiss those who aren't familar with or unwilling to use that style guide.
Does the way they've written their bid make you want to slap them, make you feel stupid or anything else negative?
Not suggesting you answer such questions here, but if you're clearer about what you need in your own head, you can probably immediately dismiss a large number of bidders.
Then you could consider dropping everyone from your shortlist who hasn't supplied a sample (or doesn't supply one on request).
And when you've glanced at their profiles, you can probably remove even more people from your list. (That's me trying to be tactful.)
And can those on your preferred shortlist fit in with your schedule?
You should be able to reduce your list to 4-5 and simply pay those a nominal fee - assuming they agree to that.
