How much cash is needed per day in Scandinavia?

Everyone in the region use their credit cards and ATM's much more than using sheer cash. There's been talk that Norway may end up a cashless society.
When we were there 18 mos. ago, we would withdraw minimal amounts from bank ATM's. Remember that each country uses different currencies. If staying in B&B's, you might want to check if they take cards.
What I do remember is that a burger, fries and Coke at TGI Fridays is $30 per person. Ouch! 7-11's are on every corner in Copenhagen and a Coke or cup of coffee is about $4. Ouch!
To me, this is a region best visited from a Baltic ocean cruise. We really got our money's worth using the ship. Only inexpensive place in the region might be Estonia.

We visited Copenhagen for five days prior to our cruise in 2014. Our lodging was paid for through the cruise line. We did purchase meals there as well as Bergen, Norway. Norway was more expensive than Denmark.

I remember having a very simple but good lunch with each of us having a beer in Bergen and cost was $35. A beer can cost $8.

Not like Portugal where I can buy a beer for .80 Euros.

I expect trains are expensive. You should be able to find out from the railroad on the internet.

I could see spending $50-75 US dollars a day for meals. One trick is to find a grocery store, and find cheaper foods to take to your hotel room.

Tours will be expensive. However, we took a free walking tour of Copenhagen and tipped the guide about $15 US each equivalent.

I would take some cash in US or Euros as insurance. We had no problem using our US credit cards and asking for signature.
However, I had a problem using my ATM card for cash. I had to exchange my backup dollars at a bank for some cash.

In Sweden, you do not need any cash. They'd rather you used a CC (or debit, but I wouldn't) for $.50 equivalent than make them work cash. It's wonderful. I suspect the other Scandinavian countries are the same, but I have no personal experience with them.

The Nordic countries are advanced first world nations. Credit cards are widely accepted in my experience (Norway, Finland, Denmark). As Wray notes, CCs can be used for relatively small purchases. I had no problem with signature credit cards. Cash needs are minimal and when you do need local currency ATM cash machines work.

I used less than 100 EUR during the 10 days in Finland (2016). Taxi's in Helsinki took credit cards. Local bus service from the train station to the holiday village of Voukatti accepted my signature credit card on the bus. And of course stores and cafes all accepted credit cards.

Similar experience during the 2016 trip that included three nights in Copenhagen. I had to make a special effort to spend the Danish Kroner that I obtained from the CPH ATM on arrival. BTW expect very large denomination bills from the Danish ATM.

I live in Copenhagen and I almost never use cash. I go to Sweden a couple of times a year and Norway occationally and I dont recall ever getting cash for these trips. The only places I can think of where cash might be needed is on public busses if you buy a ticket directly from the driver(small denominations only) or temporary shops like flea markets.

geovagriffith admits to having no experience with Scandinavian trains but feels qualified to opine on them anyway. In fact, Swedish long-distance trains (including trains from Sweden to Copenhagen and Oslo, which are also run by SJ, the Swedish national train operator) can be extremely inexpensive if booked in advance. A ticket from Stockholm to just about anywhere can be as little as 195 kr, or a little more than $20. Considering some of the vast distances involved (which don't feel so vast because the trains run smoothly at high speed), that's a pretty amazing bargain.

Sit-down restaurants can be expensive, especially if you drink alcohol, but fortunately there are plenty of other options for keeping yourself fed: Cafes, bakeries, food trucks/carts, etc. Especially if you step away from the main tourist areas, prices are quite reasonable. During a week spent in Stockholm and Copenhagen, the most I spent for a single meal was $17.

On the question of cash: As the others said, credit cards are accepted just about everywhere and are commonly used even for small purchases (and in my experience, merchants don't generally tack on surcharges). Some exceptions include: Bus fares in Reykjavik paid to the driver (though the bus station at Hlemmur sells single-ride tickets you can use later, and they accept credit cards); one slightly odd (but very tasty) cafe I encountered in Copenhagen that accepted only cash and mobile phone payments; and pay toilets (which I mostly remember from Stockholm but I suspect are in other cities too), for which you need cash and exact change.

David said credit cards and ATM's, I'd say debit cards. I find Scandinavians, more than anywhere else I've been, use debit cards for a very large percentage of their transactions. Everywhere seems to be perfectly comfortable with your card, though about half the time they ask for a signature the rest they just hand you a receipt. I only ran into one place that didn't know how to process my American card, and that was a small shop well away from any tourist center - so I walked a block to an ATM, got cash, and went back.

KHBuzzard is right on the money. SJ trains are indeed very cheap if booked in advance and if you can travel outside peak hours.
Meals can be expensive if you only stick to the main tourist sites, but you dont have to venture far to find something cheaper. Drinks can be expensive, especially in restaurants and if you want to sit down in places like Nyhavn. But you can get the same beer or Coke in a supermarket instead of a restaurant or a 7-11.
Norway is just ridiculously expensive.
And yes, most Scandinavians use debit cards.