Hi Klaus

To some degree, it depends on what your trigger or class is doing.  

If we are talking about DMLs (inserts/updates), then the limit of batch size is 200.   So lets say that the code intent is that when an Account is inserted, create a default contact for each account.  IN this case, i would create 201 accounts to insert as that would ensure you can make it past the batch size of 200.

ref: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers.htm 
Note that earlier API versions were limited to 100

If we are talking about testing SOQL results, depending on the literature you read, its either 200/500/2000.
ref: https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_changing_batch_size.htm

For myself, i do 201 records, as it achieves the DML requirement and the minimum SOQL requirement.  Also in the above link , note about when querying custom fields in object (limited to 200).

As for use of the Test.start and Test.stop.  Don't use them in the Test Setup method.  What they do is reset the Limits for the Test run.

So your Test setup should simply insert test data, and then in you Test Method, when you are invoking the code that you want to test, Test.Start() just before you invoke that call so that the DML limits are reset to zero so that you are testing that code will run in a live situation. 

For example, you have a lot to do in your test setup and you write 20 records to setup your test data.  If you did not use Test.start() , when you invoke the code, you will already have 20 DMLs, so your test would not directly mimic the real situation where the code is invoked from scratch.

Note also, that you should then use the Test.stop() before you query your test data to perform assets, because, among other things, ensures that all the DMLs in the test method are completed before you query your test data and therefore can be assured that the assertions will run correctly.

A basic structure for the use of test.start/test.stop would be as below.

@IsTest
private class someTest(){
  
  @TestSetup
  static void dataSetup(){
    //insert a heap of test data
  }

  @IsTest
  static void someTestMethod(){
    //get some of the test data that was inserted
    Test.startTest();
    //invoke the method we are testing using the test data
    Test.stopTest();
    //do a query to get the updated test data and then do some asserts
  }
  @IsTest static void someTestMethod2(){
    //get some of the test data that was inserted 
    Test.startTest(); 
    //invoke the method we are testing using the test data 
    Test.stopTest(); 
    //do a query to get the updated test data and then do some asserts 
  }
}

HTH
Andrew