If you read the design diary of Lost Legacy (a game designed to be in the same system, from a games mechanics perspective, as Love Letter) you'll note it's pretty similar. It also has explicit rules on how to incorporate more players (because it's a series of games and there are more)
The Lost Legacy "MegaMix Set"
This variant allows up to six players to play the game. Use the following guidelines to create a set of 31 cards:
- Use all the cards in this set (16 cards) and one expansion set (16 cards again. Expansions for Lost Legacy are essentialy stand alone games. Each it's own variation on the Love Letter formula)
- Remove one of the Lost Legacy cards (Princess would be the closest analogy)
If players with, it's perfectly possible to play with a Megamix Set even with only 2-4 players.
Lost Legacy Rulesbook Page 10
For Love Letter you could simply combine two decks and remove the extra princess and you should be able to fit six players comfortably. If you enjoy Love Letter might I suggest the only SLIGHTLY more advanced Love Legacy series. It takes the same mechanics of Love Letter and makes the game a little less repetitive plus it also also expansions that add even more spice to the game beyond mere cosmetic changes as we see so often in Love Letter sets.
Love Letter is at the end of the day a micro game that probably shouldn't expand the player count because a lot of what makes the game works is the 4 player cap. With more players the tension in target selection is changed. The despiration to knock out a player can get replaced with the hopelessnes of knowing there are too many players to knock them out. It's a very tight and very well designed microgame.