FIR filter : how many taps do I need ?

With more taps, you get a better correction precision in lowest frequencies. But a too precise correction may be offsetted with varying parameters : temperature or humidity, opened/closed doors or windows, moved furniture, aso… So practically, for a full range loudspeakers, 2048 taps is a minimum but I don’t think that more than 16384 taps would really be usefull. If you only use FIR eq, with no IIR parametrics, I would generally recommend between 4096 and 8192 taps. And if you only have 2048 taps or less available, better use a minimum phase correction.

And with most FIR processors, you can add some parametric EQ to finely ajust the lowest frequencies.