You can find this information on the airline websites. There is no standard across the industry, it depends on the airline. And the ticket/fare type, and also the route, even a single airline this can vary by the route- traditionally airlines have had different rules on luggage for flights to/from North America than flights within Europe, for example.
Many airlines, it is by default both a piece AND weight limit, you cannot combine multiple pieces. This started with low-fare airlines but it is has spread now as a standard to full service airlines and all three of the airlines you mention the default limit on the cheapest fares that include luggage is only 1 piece of luggage, and you cannot spread the weight over multiple pieces.
You often have the explicit option when booking if you want to book one bag, two bags, etc. So what you end up with will be exactly what you pick. But if you really want two bags, you need to make sure you book two bags when buying the ticket.
Alternately, I have done the trick suggested by @CSM of tying together two pieces of luggage to make them one piece many times.
For the specific airlines you mention:
The lowest fare for Eurowings is the BASIC fare, which includes no checked baggage.
As a saver fare, our BASIC fare includes the flight and one free piece of hand luggage.
https://www.eurowings.com/en/information/our-fares/basic.html
These are the luggage rules for the other fares on Eurowings. As you can see it depends on the fare but the SMART fare it is ONE piece up to 23kg only. Fares above that give you two items.
What charges apply for checking in luggage at Eurowings?
You will be charged for each item of luggage that you check in. If
you check in more than one bag weighing 23 kg, an additional fixed
charge for excess luggage applies. ...
Luggage can be registered at the time of booking the flight or at a
later date.
SMART fare Passengers who have booked the SMART fare can check in one
item of luggage weighing 23 kg (no special or excess luggage) at no
extra charge.
BEST fare Passengers who have booked the BEST fare can check in two
items of luggage weighing 23 kg (no special or excess luggage) at no
extra charge.
BIZclass fare If you have booked the BIZclass fare, you can place two
pieces of luggage of 32 kg (no extra or excess baggage) at no extra
cost.
https://www.eurowings.com/en/information/news-help/faq/what-charges-apply-for-checking-in-luggage.html
For BA, their cheapest fare has zero bags:
If you are travelling on a hand baggage only (Basic) fare, checking in baggage costs extra.
Beyond that, the standard is weight quoted applies to each bag, so it depends how many bags you have booked. The lower fares include only one:
The weight limit applies to each bag. It's not possible to split the total weight across multiple bags.
https://www.britishairways.com/en-gb/information/baggage-essentials/checked-baggage-allowances
For Lufthansa, within Europe, the lowest option, "Economy Light" has no checked baggage at all. The next option, Economy Classic & Flex, allows you "1 item of baggage up to 23kg". Business Class allows you "2 items of baggage, each up to 32 kg".
The rules from intercontinental are less restrictive, but the most basic "Economy Class" is still only "1 item of baggage up to 23kg". You need Premium Economy to get "2 items of baggage, each up to 23kg".
https://www.lufthansa.com/us/en/free-baggage-rules