What's the maximum ISO I can use?

The normal ISO range is from 100 up to 1600 – or even higher on some cameras. A high ISO value (e.g. 800, 1600 or higher) means a high sensitivity to light. This helps in low-light situations where you need the camera to capture more light for a better-exposed image. This is particularly true when other ways of capturing more light aren’t available to you – such as a larger aperture size, or a slower shutter speed.

Be aware that higher ISO values affect image quality. Shooting at high ISO values can cause your photos to appear grainy. In digital photography, we refer to this grain as “noise.”

Refer to this guide to know the maximum acceptable ISO value for your specific camera:
https://fstoppers.com/education/do-you-know-maximum-acceptable-iso-value-your-camera-338275

Follow the links below for a detailed guide on ISO: 

• Digital Camera Mastery - ISO - Get Great Exposure Even in Dark Conditions
• Photography Pro Website - Understanding ISO: How To Take Better Photos In Low Light