Equivalent to Raster Calculator to calculate multiple rasters (more than 2)

Have you thought about importing arcpy functionalities by using a pythoncaller? That would be one way to do mathematical operations on several rasters...

Yes, it seems the cell calculators only work on two raster features at a time. However, this does not mean you need to stack multiple transformers (or at least, not as many as you'd think). The Group-By option means you can feed as many rasters as you like into one transformer, provided they are identifiable in groups of two. You could easily create such groups using the ModuloCounter transformer.

Basically, if you start with eight rasters, with this technique you could use only three transformers (rather than eight).

Or, as @jneujens suggests, you could merge a number of single-band rasters together into a single raster feature with multiple bands, then use the RasterExpressionEvaluator to sum them together. That's probably the more elegant solution, to be honest.

You can resample rasters with the RasterResampler transformer and reproject them with any reprojector transformer too.