Because of possible inter-sample overs in the upsampling filters. This happens especially when source data contains digital clipping and the clipped peak is then partially recovered. Recent RedBook/Tidal material certainly tends to contain lot of digital clipping, thanks to loudness wars!
Data path between HQPlayer and a DAC has limited value range. 0 dBFS representing highest sample value possible. If you set volume control in HQPlayer higher than -3 dBFS, it is likely that values exceeding 0 dBFS are encountered. This is not a problem for HQPlayer’s internal data path which practically cannot clip, but it cannot be represented to the DAC using the available value range, so it would clip when producing output values. To prevent nasty clipping, HQPlayer is monitoring output and has soft-knee limiter which is triggered when such values are encountered. This causes the “Limited” counter in HQPlayer main window to increase and also volume knob changes to red to indicate “turn down the volume!”. But it is best to avoid this limiting to kick in by keeping volume set to lot enough value that there’s enough digital headroom in the output sample value range.
There’s certainly material where even -3 dBFS setting is not enough, but for most content it is enough.
Chord’s volume control cannot solve running out of sample value range before the DAC…
There’s no fixed number of taps, because there’s no fixed conversion ratio, number of taps depends for example on conversion ratio. But for example if you go form RedBook to DSD512 using closed-form, you have about 8 million taps.