This depends entirely on the configuration of the existing service with your current provider.
For example, we would be unable to port a main billing number, and leave associated numbers with the current provider, as these will stop working once the port has completed. Alternatively, we can port associated numbers, but leave the main billing number in service with the current provider.
In most cases, we are unable to split DDI ranges and port some and leave/cease the rest of the numbers. If you only wish to port part of a range, then you will need to discuss this with your current provider to see whether they can split the range for you, prior to the port taking place.