Guest count is a starting point, not the answer
The question we hear most is how many speakers a given number of guests needs. It is a fair place to start, but two events with the same guest list can need completely different systems. A hundred people in a carpeted banquet hall and a hundred people on an open lawn are not the same job. The room, the type of event and how loud it needs to get all matter as much as the headcount.
Roughly 100 guests
A birthday, a small reception or a society function of around a hundred people is usually comfortable with a pair of good powered speakers on stands, a subwoofer if there will be dancing, a small mixer and a couple of wireless microphones. In a closed room this is plenty. Outdoors, or if the dance floor matters, the subwoofer stops being optional.
Around 300 guests
This is where a pair of speakers starts to struggle at the back of the room. A wedding reception or a sangeet at this size usually wants larger main speakers, two subwoofers, and sometimes a second pair further back so the far tables hear the same speech as the front. A proper console starts to matter too, because you will have more microphones, a DJ and speeches to balance.
500 guests and beyond
Large weddings, open grounds and concerts are line-array territory. A line array throws sound further and more evenly than a stack of ordinary speakers, which is exactly what a long room or an open ground needs — the back rows hear clearly without the front rows being blasted. At this size you also want an engineer at the console for the whole event.
What changes the answer
Open air needs more than indoors, because there are no walls to reflect the sound back. Speech-heavy events — conferences, award nights — need clarity more than volume. A dance floor needs low end. Hard rooms with marble and glass need fewer, better-aimed boxes rather than more of them. The honest way to size a system is to tell us the venue, the date, the crowd and what is happening, and let us work it out.