It usually depends on your budget.
Many places will provide a generic remap designed for the type of engine for a certain fee (usually £200-£400). This map is a standard program designed for use on all engines of that type and will usually work well. However, as each individual engine can be slightly different out of the factory and also because every engine can wear differently due to use etc, you may not get absolute optimum performance from a remap of this kind.
Also, as these standard remaps are usually set up for a completely standard car, if you have already carried out some mods to the engine beforehand, a standard remap will not be suitable.
The way to get absolute optimum performance for your particular car's engine is to have a remap tailored to you by the supplier.
This usually involves a rolling road and a full diagnostic run being done on the car to start with to find out exactly how the engine is running and then, depending on your requirements, a remap is tailored for your needs.
As you can imagine, because this generally takes a lot longer and involves more work, the cost is usually much more (circa £500+).
However, if you can afford it, the results are usually much better.
J