Can a cashier refuse to take coins if there’s too many

Not if you want to keep your job. Money is money.

I have been behind people in line that paid with a ton of change. Its annoying I'll admit, but its something you have to deal with now and then. If I saw a cashier deny service to that person, I would be pissed. Maybe that is all the money they have. Either way, in customer service you do your job in a courteous manner with a smile on your face. Do not show frustration, just count as fast as you can. Do not refuse service to a paying customer.

I dunno, but money is money.

If it was an excessive amount, yeah, I'd probably say no even though I'm sure there's some kind of rule against it. I've had people pay with a bunch of change before, never an exceedingly ridiculous amount though. I'd probably tell them to go to the bank first. People know better than that.

Private businesses can refuse service to any person for any reason, unless the business is discriminating against a protected class such as race or color.

Here the shopkeepers will refuse to accept payment in coins.

If it's legal currency, then yes, you're supposed to accept it. You can turn it down if they're trying to use a 100 dollar bill to pay for an item that's ten dollars or less. Well, at least that's how it is where I work. (Though in that case we just get a lot of people who try and scam us by saying we shorted them 10-50 bucks. They seem to forget we have cameras )