When is a player less / more than a single player?

Every game style has an ideal number of players.  Just like a "buddy cop" movie is best with two players, so are games which involve a lot of witness interviewing, clue hunting and suspect timetabling.  Three is doable, sure, though sometimes one player may need to be quiet so that the other two can grill a suspect with greater precision and, yes, a single player could certainly be up to the task though they'll need a little more verbal guidance and hinting from the GM since they won't have any other mental assistance in the game.

Naturally a game of high adventure and wacky hi-jinx works well with the usual D&D group of four so that each niche can be filled and each player has opportunities to bounce off the other characters during the long stints without others in the group.  A dungeon crawl can cope well with 5 - 6 players, at most, though only if they know the rules well and won't make each round last a terribly long time.

So with all these facts in mind, on top of the GM's own needs and skill, each game will have an ideal number of players.  Having said that, some players take up more, or less, of the spotlight, time and energy within the game.  Therefore, in terms of seats at the game table, some players are worth more, or less, than a single player.

The half a players are the quiet ones.  They may have stellar moments occasionally but generally they're content to watch.  You could put eight of these around a table intended for four and so long as they know the rules well enough you wouldn't have much trouble running the game.

The three-quarter players make excellent sidekicks.  Even when they're in the spotlight, they tend to be in it to transfer it to someone else.  Rather than engaging much with NPCs or decision making, they instead serve to reinforce another player's decisions and reflect the other characters in a way that serves to gift them more limelight.

The single players are your usual run-of-the-mill player.  They like the spotlight but are happy to share.  They draw attention but not too much.  They're happy to roleplay but they generally won't slow things down to do it and when they make decisions it's generally in the best interests of the group. 

While a dedicated roleplayer whose quieter than most could be worth only one seat at the table, generally more roleplayers take up more than one.  Why is this?  Because those who are eagerly playing their character will want more time to talk, more opportunities to engage with plot, more chances to grow and develop their character, and more descriptions that emphasise who they're playing.  Don't get me wrong.  A hearty roleplayer isn't the only sort who can take up more than one seat but they tend to be mostly from this side.

When you get four hearty roleplayers at one table, each needing as much time and having as many opinions as two regular players, you really have a table of eight.  The benefit is that having a group of "eight" drawn from four players often means that you can sit back and let them "have at it".  You'll have plenty of time to think up the next part of the plot as they interact with each other.  The trouble is that steering eight players is like herding cats, especially if each cat has its own opinions.

Of course, if you have several hearty roleplayers you're probably best off looking at running smaller groups anyway.  If it's Pathfinder, two to three players aren't going to fill every niche but that shouldn't be a problem because you can simply tailor the encounters around the group's failings.  Odds are the game will revolve around decisions, antics and character arcs anyhow with combats relegated to an amusing and strategic mini-game designed to up the ante.