You will only have problems 'if' , the original seed was an F1 variety or , you had other brassicas in flower at the same time as the cabbages .
Cross pollination can only occur when plants are in flower at the same time .
So you could be growing 200 different varieties but only allow one to flower .
The seed will then be that variety .
It is best to get the seed from as many plants as possible , on a small scale 20-40 separate plants , that show the best traits of the particular plant variety you want the seeds of .
If you save seed from one or two plants that bolt , you may well be creating a strain of that variety , that will go to seed too early .
As a rule it is easy to do , you just need to follow a few very basic rules to ensure variety continuation .