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I have a method with 195 if's. Here is a shorter version:

private BigDecimal calculateTax(String country, BigDecimal amount) throws Exception 
 if(country.equals("POLAND"))
 return new BigDecimal(0.23).multiply(amount);
 
 else if(country.equals("AUSTRIA")) 
 return new BigDecimal(0.20).multiply(amount);
 
 else if(country.equals("CYPRUS")) 
 return new BigDecimal(0.19).multiply(amount);
 
 else 
 throw new Exception("Country not supported");
 

I can change if's to switches:

private BigDecimal calculateTax(String country, BigDecimal amount) throws Exception 
 switch (country) 
 case "POLAND":
 return new BigDecimal(0.23).multiply(amount);
 case "AUSTRIA":
 return new BigDecimal(0.20).multiply(amount);
 case "CYPRUS":
 return new BigDecimal(0.19).multiply(amount);
 default:
 throw new Exception("Country not supported");
 

but 195 cases is still so long. How could I improve readability and length of that method? What pattern would be the best in this case?

Create a Map<String,Double> that maps country names to their corresponding tax rates:

Map<String,Double> taxRates = new HashMap<> ();
taxRates.put("POLAND",0.23);
...

Use that Map as follows:

private BigDecimal calculateTax(String country, BigDecimal amount) throws Exception 
 if (taxRates.containsKey(country)) 
 return new BigDecimal(taxRates.get(country)).multiply(amount);
 else 
 throw new Exception("Country not supported");
 

Put the data in an XML file or database, then use it to populate a dictionary. That way you can change the data easily, and separate the data from your application logic. Or, just execute a SQL query in your method instead.

If the values are constant and are not meant to be changed regularly (which I doubt). I'd introduce a static metamodel using Enum:

public enum CountryList 

 AUSTRIA(BigDecimal.valueOf(0.20)),
 CYPRUS(BigDecimal.valueOf(0.19)),
 POLAND(BigDecimal.valueOf(0.23));

 private final BigDecimal countryTax;

 CountryList(BigDecimal countryTax) 
 this.countryTax = countryTax;
 

 public BigDecimal getCountryTax() 
 return countryTax;
 

 public static BigDecimal countryTaxOf(String countryName) 
 CountryList country = Arrays.stream(CountryList.values())
 .filter(c -> c.name().equalsIgnoreCase(countryName))
 .findAny()
 .orElseThrow(() -> new IllegalArgumentException("Country is not found in the dictionary: " + countryName));

 return country.getCountryTax();
 

Then

private BigDecimal calculateTax(String country, BigDecimal amount) throws Exception 
 return CountryList.countryTaxOf(country).multiply(amount);

It's readable, compile time safe, easily extendable with additional metadata per country and less boilerplate.

You may consider creating two arrays. It is easy to implement. For instance, in this case you may use :
String[] countries = "Poland", "India", "Mexico",....;
And another array for the respective BigDecimalvalue.
BigDecimal[] taxrates = 0.23,0.5,0.75;
Notice that since both the arrays are parallel, the value of index for both the arrays would be same for whichever is selected. For instance, index = 1 would always point to India and 0.5.
In case you need to remove or add an element from the array, you can use a temporary array and copy the values. Then create a new array and copy the required values. You can also edit values using index like using SQL id. In case of dynamic and drastic changes I suggest using ArrayList where we can easily access get() and indexOf("") or remove("") methods.
There maybe better approaches but I personally find this easy to implement.
P.S. - Hashmap may also be used to solve the problem.