Title: I (22M) don’t believe my girlfriend (21F) about her body count and I can’t stop overthinking it. How do I know if she’s telling the truth?

I’m 22M from Ireland and my girlfriend is 21F.
We had a situationship for around 3–4 months before we started officially going out. We had both gotten out of serious relationships around the same time, and not long afterwards we started seeing each other.
We met on Tinder and talked on Snapchat for about a month before meeting. We went on one date and got on really well. We kissed on the second date and had sex on the third.
Everything was good. She was comfortable having sex as long as I wore a condom.
She loves nights out. While we were still seeing each other casually, she later admitted that she never actually met up with other guys, but she would regularly make out with guys on nights out. According to her, that’s where it stopped.
One day, just before we became official, she asked me my body count. I said 6 and asked hers. She said 4. At the time I assumed that was me, her two exes, and someone else. I didn’t really care.
We’ve now been officially going out for about a month, but recently I started doubting whether she was telling me the truth.
She has always told me she wouldn’t have a one-night stand. However, she recently told me that around the time we first started talking, she got very drunk after a club, got a taxi back to a guy’s house, went inside with him, but then changed her mind, turned around and got a taxi back into the city centre, met her friends and eventually went home.
She said one of the things going through her head was, “Imagine what my parents would think if I didn’t come home.”
I also admitted to her that I had sex with someone else while we were still only casually seeing each other.
Eventually I told her I didn’t believe her body count was really 4. She asked what I thought it was, and I said maybe 7 or 8.
My reasoning was basically that she’s going into her third year of college, goes out a lot, has kissed a lot of guys and has had plenty of opportunities.
Then she told me that her actual body count is 3, not 4.
She seemed genuinely embarrassed and went shy when she admitted it. She said that when I told her mine was 6, she thought her number of 3 sounded too innocent, so she lied and said 4 instead.
She says she has only ever had sex with guys she knew and trusted, and I’m one of those three. She says she has kissed plenty of guys but that’s normally where she draws the line.
To be fair to her, during the months we were casually seeing each other, I never noticed anything that clearly contradicted that. I wasn’t stalking her or anything, but after nights out she always seemed to be back home the next morning.
I’ve asked her repeatedly about it and told her that I genuinely don’t care what the number is. If she told me tomorrow it was actually 20, I think I’d almost feel relieved because at least I could stop wondering.
What bothers me is the possibility that she’s lying to me.
She has sworn on her mum’s life, her granny’s grave, etc. She’s generally a genuinely nice girl, and she comes from a fairly religious family.
But because she originally said 4 and then changed it to 3, I can’t stop wondering whether there’s more she isn’t telling me.
How can I actually know whether she’s telling me the truth? Is there even a way I can ask her that would make her feel comfortable enough to be completely honest, or do I just need to accept her answer and stop asking?
TL;DR: My girlfriend originally told me her body count was 4, but recently admitted it’s actually 3 and says she only lied because she felt embarrassed that her number sounded “innocent” compared to my 6. She goes out a lot and has kissed lots of guys, so I keep convincing myself her real number must be higher. She has repeatedly sworn she’s telling the truth and says she has only slept with guys she knew and trusted. I don’t actually care what the number is — I’m just obsessing over whether she’s being honest. How do I know when to trust her answer and stop asking?