Mother is 94 and sleeps a lot. How much sleep is normal?

As much as she needs.

Did it happen suddenly or has he been gradually increasing his sleep or a while?

If it's more than 24 hours a day, then it's time to worry.

If I was 94 I'd sleep a ton too.

My Dad is 93 this month. He sleeps 18-20 hours a day. If he has an outing, he sleeps more the next day.

He still manages to prepare his meals and prior to Covid did his shopping,

What do you mean by a lot ?