VMs only use the RAM when it is running.

With 24 GB of RAM you should have plenty to spare, you would have to monitor your RAM usage to see how much you normally use, you make no mention of what OS your host PC is running.

Kali Linux has this to say in it's documentation:

On the low end, you can set up Kali Linux as a basic Secure Shell (SSH) server with no desktop, using as little as 128 MB of RAM (512 MB recommended) and 2 GB of disk space.

On the higher end, if you opt to install the default Xfce4 desktop and the kali-linux-default metapackage, you should really aim for at least 2 GB of RAM and 20 GB of disk space.

When using resource-intensive applications, such as Burp Suite, they recommend at least 8 GB of RAM (and even more if it is a large web application!) or using simultaneous programs at the same time.