I would say 4 if you can deal with the heat issue. For Madevallia, you can probably go with 3.
At 1' (30cm), it is probably around 190micromol/m^2/s (1500fc) from my measurement. But this is the standard 4-bulb fixture. If you are spacing them further (which you should), then it is lower. But it is still more than plenty for typical Masdevallia.
If you remove bulbs from a bigger fixture, it doesn't linearly reduce the energy consumption (in other words, you could get really low system-wide efficiency). I don't know well about this, but it is probably something to do with 1 ballast driving a couple bulbs. Better fixtures (e.g. Hydrofarm) have switches to use only a half of the bulbs (then the efficiency shouldn't be affected).
Ray, I think Ron is using 4x T5HO according to the comments there. But he grows plants requiring much more light than Masdevallia.
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