I bought 50gb iCloud storage,it it just uses about 8.8 gb,but the system keeps displaying insufficient storage

Does it say you have insufficient storage on your iPhone or in iCloud? It is not possible to buy more storage for an iPhone; it will never have more than it did when you bought it. What you bought is iCloud storage that is used for backing up and also for syncing to other devices. Here is an explanation→What's the difference between device storage and iCloud storage? - Apple Support

iCloud+ has 2 independent functions. iCloud backups, and iCloud sync. It’s confusing because they both have the same name prefix. But they have no connection with each other. Go to Settings/[your name]/iCloud. You will see a bunch of switches (with iOS 16 also tap Show All). When you turn on a switch that data type will sync to iCloud. If you do this on multiple devices that share an Apple ID the selected data will sync to all of those devices. 

You can also turn on iCloud Backups. This will back up your phone every night if the phone is plugged in, connected to Wi-Fi and locked.

But note that anything that you sync by the first method will be excluded from the iCloud backups.

When you get a popup message on your iPhone or iPad that the storage is full.

Look closely at the storage of the iPhone and iCloud

iPhone Storage (Setting > General > iPhone Storage)

1. Total storage remains the same throughout the life span of the iPhone
2. Storage can not be increased
3. You may take a backup on Mac or PC using iTunes / Finder

iCloud Storage (Settings > [Tap your name] > iCloud)

1. By default, every user gets 5GB of free storage space
2. You can buy more storage based on your requirement 50GB, 200GB, or 2TB with a monthly subscription to Apple One you may get more storage.
3. Based on your storage capacity you can back up your iPhone or iPad

The iCloud storage space can be shared between your family members

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.