Since I haven't read the books, I'd like to know exactly how unfaithful she's been over the course of the books and the TV series. From what I can recall off the top of my head from the TV series, she's been with:

• Robert Baratheon


• Jaime Lannister


• Euron Greyjoy


• Lancel Lannister?

What I'm curious is whether or not her infidelity shown in the TV series matches the books, or is it an overestimate or underestimate?

Game of Thrones

In the show she has been with Jaime Lannister as we see in the very first episode. She has also slept with Robert and was even pregnant by him

Cersei Baratheon: I felt something for you once, you know?

Robert Baratheon: I know.

Cersei Baratheon: Even after we lost our first boy -- for quite a while, actually. Was it ever possible for us? Was there ever a time, ever a... moment?

_{Game of Thrones, Season 1 Episode 5, "The Wolf and the Lion"}

She has of course been with Lancel Lannister too.

TYRION: Have you ever given any thought to what King Joffrey will have to say when he finds out you've been bedding his mother?

LANCEL: It's not my fault!

[...]

TYRION: Did he tell you to fuck her, too?

LANCEL: I only meant I did as I was bid.

_{Game of Thrones, Season 2 Episode 4, "Garden of Bones"}

Then of course she fucks Euron Greyjoy in Season 8 in return for his loyalty.

A Song of Ice and Fire

In the books she also sleeps with Robert, Jamie and Lancel but not Euron. However she also sleeps with Osney, Osfryd and Osmund Kettleblack at some point.

Cersei could hear Unella writing behind her. Her quill made a faint, soft scratching sound. "Lancel Lannister, my cousin. And Osney Kettleblack." Both men had confessed to bedding her, it would do her no good to deny it. "His brothers too. Both of them." She had no way of knowing what Osfryd and Osmund might say. Safer to confess too much than too little. "It does not excuse my sin, High Holiness, but I was lonely and afraid. The gods took King Robert from me, my love and my protector. I was alone, surrounded by schemers, false friends, and traitors who were conspiring at the death of my children. I did not know who to trust, so I … I used the only means that I had to bind the Kettleblacks to me."

"By which you mean your female parts?"

"My flesh." She pressed a hand to her face, shuddering. When she lowered it again, her eyes were wet with tears. "Yes. May the Maid forgive me. It was for my children, though, for the realm. I took no pleasure in it. The Kettleblacks … they are hard men, and cruel, and they used me roughly, but what else was I to do? Tommen needed men around him I could trust."

_{A Dance with Dragons, Cersei I}

It's probably said in more of a ficticious manner but she potentially also slept with Moon Boy.

"You poor stupid blind crippled fool. Must I spell every little thing out for you? Very well. Cersei is a lying whore, she's been fucking Lancel and Osmund Kettleblack and probably Moon Boy for all I know. And I am the monster they all say I am. Yes, I killed your vile son." He made himself grin. It must have been a hideous sight to see, there in the torchlit gloom.

_{A Storm of Swords, Tyrion XI}

She also sleeps with Taena Merryweather in the books, although she is a woman, obviously, it is worth a mention.

Cersei cupped the other woman's breast. Softly at first, hardly touching, feeling the warmth of it beneath her palm, the skin as smooth as satin. She gave it a gentle squeeze, then ran her thumbnail lightly across the big dark nipple, back and forth and back and forth until she felt it stiffen. When she glanced up, Taena's eyes were open. "Does that feel good?" she asked.

"Yes," said Lady Merryweather.

"And this?" Cersei pinched the nipple now, pulling on it hard, twisting it between her fingers.

_{A Feast for Crows, Cersei VII}