Hi evelyne132,
welcome to the Apple (user-to-user) Community.
Re: Missing Photos: "I’m noticing they are lost.
No one deleted them or hide them".
It is always wise to keep a back-up of any files / data that is important, or valuable to you. Also, to keep your iOs version updated to the latest one.
The photos may be available for recovery in the "Recently Deleted" Album. Apple Support's article:
What to do: If you're missing photos or videos in the Photos app
Eg: says:
"Go to Photos > Albums,
and tap Recently Deleted in the Utilities section".
(See the article for how to recover the photos, if you find them ok.)
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Re: "Help me understand how that is possible"
You might want to check you have enough storage space on your iPhone for all of your photos: ie: How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
Apple Support's article:
If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support
says,
How to: "Manage space on your iPhone, iPad, or iPod touch"
Also: How to: Manage your photo and video storage
and
If you use iCloud:
What's the difference between device storage and iCloud storage? -
Apple Support
If convenient / needed:
You might want to access old (or new) files from an external storage device, (eg: zip or hard drive).
How to: Connect external storage devices to iPhone – Apple Support
More support, if needed:
Photos Support Website
There's a "Get Support" link at the bottom of their webpage:
(online chat, phone eMail etc.) link below.
Photos - Official Apple Support
All the best :-)