I have Mojave 10.14.6 and would like to know if I can uprade to Ventura

See the following from Identify your iMac model - Apple Support

iMac (Retina 4K, 21.5-inch, Late 2015)

Model Identifier: iMac16,2

Part Numbers: MK452xx/A

Newest compatible operating system: macOS Monterey

A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. 

See instructions in: How to download and install macOS - Apple Support  

Please refer to macOS Ventura is compatible with these computers - Apple Support

Looking through it, no your 2015 iMac it simply too old for Ventura. If you have a need for Ventura (or beyond) then its time for that new Mac you have been wanting!

Monterey is still one of the "most recent three" – so there probably aren't too many applications that would run on Ventura, but not on Monterey.

If you upgrade to Catalina or later, 32-bit applications will stop working. iTunes will go away, replaced by separate applications (Music, TV, Podcasts, Books, and Finder-based iPhone/iPad/iPod management).