You can activate 4 MM addresses to get 15, 16, or 17 functions or so, depending on the decoder.
When setting CV 49 to 3, there should be four sequential addresses.
When setting CV 49 to 7, you can set 4 addresses independently.

With value 7, second address is CV 75, third address is CV 17 and fourth address is CV 18.
With value 3, the train should respond to addresses 51, 52, 53, and 54.

Do you know someone with a Central Station or Mobile Station? In the past I encountered problems activating all four addresses with the 6021, but once activated with a Central Station all four addresses could be used with a 6021 and even with a starter set controller that only has 4 addresses in total.

I think f0 through f16 is the maximum you can get with Märklin decoders.
With a Central Station one can easily assign function keys. Thus you can move boring sounds to f17 through f31 and interesting effects to f0 through f16, placing the most interesting function on f0 through f4.

Marklin have only ever had four MM2 addresses giving 16 (well +F0 for headlight) addresses as an extension to allow mfx items to be controlled by a 6021. Remember mfx originally had only the 16 functions, so that was all the MM2 expansion method accommodated.

The route to accessing the remaining functions is to get a cs, and the Marklin 6000 series items adapter to allow you to use your existing investment with the new system. But do remember that in terms of controlling locos from 603x controllers you will still have significant limitations on function availability, unless there is some sneaky functionality I am not aware of.

Remember the items you are now purchasing have maintained backward compatibility, but that doesn't mean you get total functionality.

Yeah, but to do that he is going to need something more than a 6021, at a minimum a 60971 with associated software running on a PC, to do the remapping, or a cs2 or cs3.

Once he starts down that road, getting an ms2 and using the loco as it comes sounds like a much more sensible idea, by the time one has learnt how to use the programmer software. using an ms2 he can use all 32 possible functions on any loco fitted with them without having to remember a matrix of 'what turns this function off or on'.

A 60971 would be a not insignificant proportion of an ms2 setup in cost, plus having a Windows PC to run the software.

Myself, had I not already had a CS3, I wouldn't have got the 39810. Glad I got a CS3 first. My 6021 I still use on a test track, and for a Christmas oval under our tree.