What did Seneca say about solitude and crowds?

Seneca does not tell Lucilius to choose solitude over society, or society over solitude. He tells him that both are dangerous for someone who has not yet mastered himself, and both can be safe or even good for someone who has.

On crowds, the warning is blunt. Seneca writes that a crowd is especially to be avoided, "for as yet you cannot trust yourself to them with safety" (Moral Letters 7.1). He admits he feels the effect on himself: he never comes home with the same character he took out. Something calmed within him gets stirred up again, "some of the foes that I have routed return again" (Moral Letters 7.1). He compares this to a convalescent who relapses if taken outdoors too soon. The danger scales with the size of the crowd. He singles out the games, the public spectacles of his day, as especially corrosive, because there "vice steals subtly upon one through the avenue of pleasure" (Moral Letters 7.2). Nobody escapes a crowd unmarked: everyone in it either makes some vice look attractive to you, presses it onto you, or leaves a trace you never notice (Moral Letters 7.2).

That is the passage people remember. It is often read as a simple rule: avoid people, seek solitude, that is the path to virtue. Seneca corrects this himself, in the very next letter cited here. Letter 10 opens with him repeating his position, "avoid the many, avoid the few, avoid even the individual" (Moral Letters 10.1), which sounds like an even harder line against company. But then he turns the warning around and points it at solitude itself. He tells the story of Crates, who found a young man walking alone and asked what he was doing. "I am communing with myself," the young man said. Seneca then makes the point explicit. People who are grieving or afraid are watched carefully, precisely so they are not left alone, because a thoughtless person left alone will only plan folly, indulge base desires, and let unchecked passions grow (Moral Letters 10.2). The one real benefit solitude offers, the habit of trusting no one and fearing no witness, is lost entirely on a fool, because he betrays himself anyway (Moral Letters 10.2). So the crowd corrupts you from outside, and an unformed mind corrupts you from inside when no one is watching at all. Seneca's actual position is narrower than "avoid people": avoid anyone, including yourself, until you are someone worth spending time with. His highest compliment to Lucilius is that he trusts him with himself, that he knows no one he would rather have Lucilius keep company with than Lucilius (Moral Letters 10.1, 10.2).

This is close to what Marcus Aurelius says about retreat, though Marcus takes it in a different direction. Marcus notes that people run to country houses and seashores for peace, when the real retreat available at any time is into one's own mind (Meditations 4.3). But Marcus's confidence that retiring into the soul restores tranquility already assumes the settled mind Seneca is warning that most of us do not yet have.

The practical upshot is not a rule about geography. It is a question to ask before either kind of retreat: is the self you would be alone with, or the crowd you would join, going to make you weaker or steadier? Seneca's own practice was to limit exposure to crowds while working, letter by letter, on becoming someone who could eventually trust his own solitude.