Those are very good questions, FME Lizard.

In short... you can have as many engines as you like. To increase (or decrease) the number of engines simply access the FME Server interface, click on Engines and Licensing, set the required number of engines, and click the Change button.

There is no extra cost involved.

This YouTube video demonstrates the steps involved:

However, while adding a large number of engines sounds attractive, it might not be a good move.

When your processing is CPU-intensive - for example you are doing geometry transformation - then the optimal number of engines is one per CPU core, and that's what we configure a default FME Cloud instance for.

With that type of transformation, if you have more engines than cores then there is not enough processing power to efficiently process each job, and it might even result in degraded performance.

So, a larger number of engines is best when the translations are more IO-intensive; i.e. each translation is waiting for Input or Output from somewhere else (maybe a disk, a database, or a web service).

In that scenario, CPU resources are not being used to their maximum, and so it makes sense to increase the number of engines to take advantage of the excess.

You can also find more information in this question about the optimal number of engines on an FME Server.