One stereo or two (stereo and mono) cartridges?

It truly depends on your personality and values. It is probably breat to hear different point of views from here. But it is really you.

Personally, I can seldom listen to a mono recording… I get bored almost immediately. I loved a Fats Waller album but am so disappointed it is not in stereo. It just falls flat. Another personal thing… I want the best possible sound at all times, so I would never own two cartridges. Never.

Alternatively I have read about folks with nearly a dozen cartridges with head shells… pairing the perfect cartridge with a particular album… I am left scratching my head.

On the other hand, I shoot a Leica M10 and have been very tempted to buy a Leica M10 monochrome (only shoots monochrome… higher resolution and more gray shades). 
 

So, it comes down to you. Given your question I like the idea of splitting the investment in proportion to how much you listen to stereo versus mono. 
 

Also, from my experience if I have a $10K budget… definitely worth spending it. My cartridge is around $8.5K, definitely well worth it. 

@ghdprentice personally, I can seldom listen to a mono recording… I get bored almost immediately. I loved a Fats Waller album but am so disappointed it is not in stereo. It just falls flat

You clearly are a seasoned vinylista, but in my experience good mono is far from boring. Ymmv

@luis223344 investigate Tedeska for Mono ( and Stereo for that matter ) 

Thank you @solypsa , I researched Tedeska and they sound very committed to

and in mono in particular. That is the fun of the hobby, research for impossible... Few seem to own mono and stereo carts. Surprises me. Michael Framer was advising NOT to buy the Beatles in Mono box set if you did not own a mono cart. Obviously, he wants to help sell hardware, and advertising..., yet his article was interesting and seemed to make sense. I bought the box set nevertheless...

There are ''implicite'' and ''explicite'' premises or ''presuppositions'' . There

is also the ''theory''  that there is no ''theory independant obesrvation''.

I Iintroduced ''presupposition'' because ''theory'' is to optimistic reg. our

presuppoitions. . ''Some'' may be ''programmed'' by our upbringing . That

is learning them  with lour anguage  acquisition . Among those there are

some about ''order'' or ''sequence''.

For example if one does not own ''mono LP''s'' why should he or she need an

mono cartridge? My question is ''deduced'' from the ''fact'' that everybody

seems to own i-phones. Because the other owns them or because one

can't do without.? But I must confess not to own one. because I already

own the ordinary kind. For, say, few calls pro week those are sufficient. 

That is then my presupposition. 

I beg to differ with "spatialking", where he states "Also note that many mono recordings are not RIAA compensated, so spending a lot of money for a mono cartridge could be somewhat of a waste." The statement has merit if you are talking about 78 rpm records from the 40s and earlier. However, once we entered the LP era in the early 50s, all LPs are frequency compensated according to one or another algorithm, and by the mid to late 50s, all LPs I know about were RIAA compensated. I guess some British companies continued to use other algorithms longer than we did in the US, before standardizing on RIAA. Moreover, frequency compensation does not take place at the level of the cartridge; this is done in and by the phono stage, so in a way the point is irrelevant. My opinion is that there is much to be gained, not just a reduction in noise, by playing mono LPs in mono and that on the other hand there is no real need to invest in a mono cartridge IF your line stage has a mono switch. Just flip the switch for mono LPs, and you will hear mono LPs with much improved fidelity, compared to playing them in stereo with a stereo cartridge. That’s just my opinion. Yes, you might squeeze out another dollop of wonderfulness if you use a mono cartridge; I am not going to dispute that, but also consider that you are then comparing two different cartridges, as well as two different processes.  Anything goes in that regard.