Horse rescue...what to do?
First off, kudos to you for volunteering your time for this good cause.
That's a lot of animals for a small area especially if the number is closer to 50. I've seen 12 horses on 6 acres turn a pasture into a mud hole. But I would expect a rescue facility to be smaller than larger when funding is always scarce. Sounds like this facility needs to cap it's number of animals that it's taking in. Not fair to the animals already there.
While it sounds like the animals are basically cared for with food and I assume water, it seems like there are some safety concerns if trash, hay wraps (which I think could be real dangerous if ingested) and metal, not too mention manure not being removed in a timely manner. I suspect the facility does not have enough help to get everything done but I don't think that's an excuse for safety issues to be present.
You can talk with the rescue facilities mgt to voice your concerns but if it's so obvious, the mgt knows about it and chooses to let things go for whatever reason (again, resources to get things done probably near the top of the list). I would be tempted to have an organization like the ASPCA assess this facility but I believe they are geared towards animal cruelty which doesn't sound like the case you are talking about here. Maybe somebody else knows of another avenue to get the facility assessed.
The animals have no voice but you can help by taking some action.