Hi,
LM386 is not a classical Opamp. According TI datasheet it is a "Low Voltage Audio Power Amplifier". It is designed to drive low ohmic loudspeakers.
Out of curiosity I did an ebay search. There is much crap around. Some data given are just unrealistic "advertising values".
Now how to get meaningful reliable values?
* find out what hip (IC) is used. Best: Manufacturer and exact type. Example: "TPA3116"
* then do an internet search for: "TPA3116 datasheet"
* the values given in those originnal manufacturer datasheets should be reliable.
You should look at
* output power (depends on speaker_ohms, supply voltage, expected distortion....). I'd say 3W should be sufficient for normal listening (not very loud). 50W will be quite loud. Don't choose the highest power, choose one that fits your needs.
* distortion. For quite good sound I recommend it to be below 0.1% at the expected loudness. The lower the better.
* Amplifier class:
- Class A, AB, B are linear amplifiers (descending quality). They may get hot, may need a heatsink at quite low output power
- Class D is efficient, quite high output power even with no or a small heatsink. The output is digital switching, it can't be used as input for other sources like "recording". They come with or without output filter (LC low pass). I'd choose one with output filter.
* Those modules need DC power supplies. Look at the datasheet which voltage to choose, don't choose one with higher output voltage!
Amperage is less critical. Too low amperage will be fine for low output power, but will cause to distortion or malfunction at high output power (loudness). To high amperage is no problem at all. Maybe you need extra capacitance maybe extra filters in case you hear some noise (hiss) at low volume.
Klaus