How do you deal with repetitive requests to go to the bathroom? Even when they are on the toilet they are asking to go. What do I say?

A toileting schedule. Every two hours or every hour, depending on need. Take them to the toilet maybe a half hour after a meal or snack. Or if you've given medication.

You have to ignore the constant, 'I have to go to the bathroom'.
No one can play the toilet game all day and night long. Enough is enough. Make sure there's no UTI's going on. Also check the person out to make sure there's no redness, rashes, fungal infections, and skin breakdown on their backsides and genitalia. This will also make a person with dementia keep asking for the bathroom.

Always make sure they're clean. Clean as in no crap left behind in the behind. This can cause itching and irritation.
Asking the doctor for some anti-anxiety medication won't hurt either.

With dementia, OCD is common. My mother was fixated with the toilet herself. Call the doctor to discuss the matter and request mild calming meds to help relax the loved one and perhaps distract them from the constant loop thinking.

Could be the urge to go is strong, yet problems voiding.
UTI? Or constipation?

Or there need is something else altogether. My mom's phrase used to be asking me to "turn her over", sometimes she really did want to be repositioned but usually those were just meaningless words that had me playing 20 questions trying to figure out why she was agitated.

If this person is male, get a medical appointment to check for prostate enlargement. Medications can help with the "urge" from benign prostatic hypertrophy. Increased "urge" can also be a sign of infection or cancer.

If this person is female, get a medical appointment to check for urinary tract infection.

Yes, the toileting becomes very problematic.
Part of this is mental with toilet training being something that "sticks with us" in early stages of dementia, and there is fear of loss of control.
But there are also physical problems of bladders holding less before they begin to trigger the need to void, and with incomplete empty.

This urge can also happen in urinary tract infections. Keep dip sticks for urine testing, available on Amazon, and check urine frequently for any sign of nitrites or leukocytes, and take specimen in for Culture and Sensitivity testing to rule out signs of infection. See MD for this complain and get input, also.

Clarkkentslady: An individual who unfortunately suffers from dementia may often be fixated on toileting issues. However, the person should see their physician to rule out any maladies.

If a person is incontinent a schedule won’t help.

During my mother’s last years, she would go to the bathroom and turn around and have to go again. She had no control over her bladder.

Trying to make a bathroom schedule work was impossible. It only caused stress for her and for me.

Mom and I both decided that wearing depends all the time was the best solution.

Obsession over toileting is actually common. when my mom was still able to live alone, she spent half of her day on the toilet. She didn't really need to go, it was just something she did. She flushed all kinds of things down the toilet. I had to take the cardboard cores out of the rolls, hide paper towels and other things, it was a nightmare. You can imagine how often she overflowed the toilet.

Confer with PCP. Get some level of in home caregiving services on board; not only will this help you greatly,they can provide additional education, input on coping with such issues. Have PCP refer a Geriatric Case Manager to the case. They can assist with options also from multiple perspectives.