FIR filter : how many taps do I need ?

With more taps, you get a better precision for lowest frequencies correction. But a too precise correction may be spoiled by varying parameters : temperature or humidity, opened/closed doors or windows, moved furniture, aso… So practically, at 44.1 or 48kHz and for a full range loudspeakers, 2048 taps is a minimum but I don’t think that more than 16384 taps would really be usefull or even recommended. 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, it is much better to use a minimum phase correction.

And with most FIR processors, start with some parametric EQ to ajust the lowest frequencies and remeasure to calculate the FIR correction.

Remember that for higher sample rates, you have need proportionally more taps : 8192 at 48kHz has same resolution as 16384 taps at 96kHz.