How long will I get security updates for MacOS Catalina?

If Apple continues, as it has been for several years now, Security Update for Catalina macOS 10.15 will continue for Three ( 3 ) year from date of Release . Catalina was released October 7, 2019

Part 2 of question - the OS is good to use even after the Security Updates cease. A little more Prudence may be required. Am using a circa Mac mini 2011 with best possible OS High Sierra 10.13.6 and exercise prudence and some caution for limited Internet usages and email and messages plus iTunes.

Apple supports its operating systems based on a now customary three-year sliding window based on the release date of the operating system, not the purchase date of the Mac, because someone may have upgraded their Late-2013 iMac to Catalina. That three-year window means that Apple will drop support for Catalina at some point in Fall 2022. That timing will be Apple's decision, something we won't know here.

The impact of Apple dropping support for an operating system means no further security, browser, and in some instances Apple application updates. Your Mac will continue to function without these updates, but at some point, Apple and third-party applications will require a newer operating system for their installation. Eventually, Apple's Gatekeeper security software won't be updated to scan new application installations for malware either.

Microsoft tracks Apple's support window, so in Fall 2022, Microsoft will then require macOS Big Sur to install MS Office, and AutoUpdate will cease to provide MS Office updates to Macs still running Catalina. That includes Microsoft 365, and either of the current Office 2019/2021 perpetual license suites.

The other factor is that Internet security marches on, and at some point in the next few years, Apple's Safari browser on Catalina may begin to experience connectivity issues with those websites implementing newer security protocols. This can be circumvented by using the then-current version of Firefox, or the Brave Browser.

Welcome and Thank you too.

Do come back to the ASC Forums, in the future, if / or when additional question may arise

Very Best in the New Year !

Option exist if one were is the Market for new Apple Computer and no not up selling.

Depending on Region Turn the device you have into the one you want. Have used this method 3x in 2020 up upgrade to new iPhone, M1 Mac Mini and M1 MBA and have been satisfied with the outcome. Plus, save the Environment by the old units being recycled.

Also, depending in Region Refurbished Mac and yes, have personally used this method over the last years also with very good results.