there is no definite rule for deciding the number of stripes.
Actually the multiple stripe concept work as below.
When multiple stripes are used, database dumps are written to all the devices paralelly and equally.
For instance if we use 5 stripes to take the dumps, on OS level 5 files will be created for the dumps.
ASE will write the dump on all the devices parallely and in equal amount, so we will see that the size of all the 5 files, once the dump is finished, will be same.
So by using stripes, we are using multi processing functionality of OS.
So we can say more number of devices, means more parallism at OS level.
But as the number of stripes increases the burden on OS for performing mutiprocessing also increases, which will again reduce the dump process speed.
So in your case as database is of 20 GB, 5 stripes (of 4 GB each) will be a good idea.
You can further reduce the size of files using compress option while taking the dumps, ideal compression level will reduce the dump size to 60% of the actual database size(its my experience, and may vary from platform to platform, also depends on ASE version).
Hope this will be helpful.