Can you play Killer with two players?

You can, but it is just an unusual game of pool at that point. The format needs at least four or five people before the rotation does anything interesting.

What is the maximum number of players?

There is no mechanical limit. Past about twenty-four on one table the gap between your turns gets long enough that people wander off, so add a second table or split into two nights.

Every Killer pool variation, and which to actually use

Killer is a folk game, so there is no official version. Here is every common variant, what it does to the night, and whether it is worth it.

Life-count variants

Three lives. The default, and right for ten or more players.

Five lives. Use it when the field is under ten, or the night ends too fast. Do not use it with twenty people unless you want a three-and-a-half hour evening.

Two lives. Brutal and surprisingly good as a quick second game when the main event finishes early. Twenty people, two lives, forty minutes.

Verdict: match lives to field size. Three for ten-plus, five for under ten. Decide before the draw.

Gaining lives back

Pot the black, gain a life. The 8 ball buys a life and is then spotted. Gives a player on their last life something to aim at, and creates genuine drama. Verdict: keep it. The best variant on this list.

Two in one shot, gain a life. Rewards a good shot rather than a lucky one. Rarer. Verdict: fine, but it fires so seldom that it mostly adds a rule nobody remembers.

Clearing the table, gain a life. Whoever pots the last ball of the rack gets one back. Verdict: avoid. It rewards whoever happens to be in the rotation at the right moment, which is luck without any skill attached.

Penalties

Scratch costs an extra life. Very common, especially in the UK. Sharpens the game considerably. Verdict: keep it, and announce it. This is the single most argued-about rule in the game.

Ball off the table costs a life. Usually bundled with the scratch rule. Verdict: yes, same category.

Foul costs a life. Meaning any foul, including failing to hit a rail. Verdict: avoid in a bar. It needs a referee you do not have and it generates arguments at a rate no night survives.

Buying back in

An eliminated player pays the buy-in again and returns with one life, or with their original count. Common in UK pubs where the table takes coins anyway.

Verdict: keep it, with a hard cutoff. "No buy-backs after the third rack" or "no buy-backs once we are down to five". It grows the pot and keeps people in the room, and without a cutoff it makes the ending drag indefinitely.

Announce the cutoff at the start. A buy-back negotiated at the moment somebody is eliminated is a different and much worse conversation.

Structural variants

Keep shooting until you miss. Instead of one shot each. Turns the game into something much closer to real pool, and much less even — a strong player clears half a rack while everybody watches. Verdict: avoid for a bar night. It undoes the main reason the format works.

Teams. Pairs share lives and alternate shots. Verdict: good with an odd mixed-ability crowd, and it halves the effective field size, so add lives.

Nominated ball. Each player must pot a specific ball. Verdict: avoid. Too hard, too slow, and strikes pile up for reasons that feel arbitrary.

A recommended ruleset

If you want one that just works in an American bar with twelve to twenty-four people:

Three lives
One shot each, table passes on regardless
The break is free — break, then take your shot
Scratch costs an extra life
Pot the black, gain a life, and the black spots back up
Slop counts
No late entries, no buy-backs

Announce all seven in about twenty seconds before the first break, every week.