I have the El Capitan OS. How much can I update it?

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

If no ” Get Button “  Replace the https in the page's address to macappstore. Credit to Contributor @Niel

Computer Qualify for the Monterey upgrade

macOS Big Sur 11 hardware requirements

macOS Catalina 10.15 hardware requirements

macOS Mojave 10.14 hardware requirements

macOS High Sierra 10.13 hardware requirements

macOS Sierra 10.12 hardware requirements

Caution - jumping over all the newer versions and directly to Big Sur  / Monterey on Qualifying computers is Technically possible - there are those who have attempted and were successful. Then there are those who attempted and had less than good results.

If this is a Production or Mission Critical Set-up - a more prudent and measure approach is suggested. 

The above is from observations on the Apple Support Community Forums and IMHO

Tips Big Sir Upgrade Released Nov 2020

Tips Monterey Upgrade  Released Oct 25, 2021

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Back up your data, check your applications for compatibility, and if your Mac model is recent enough, click on High Sierra, Mojave, Catalina, Big Sur, or Monterey. If you get a Mac App Store preview page, change the https at the beginning of its address to macappstore.

Catalina and newer won't run 32-bit software; download and use Go64 to scan for it. To look up other programs, click here.

(215008)

Rotational Drives and especially Fusion Drives will take a performance Hit ( negatively ) on Catalina macOS 10.15.

Factor in Catalina only supports 64 Bit everything and there are no workarounds for this.

Wherein Mojave supports both 64 and 32 Bit Applications and is less demanding.

Both Catalina and Mojave have reached or about to reach end of Security Updates.

You are welcome and Thank you too.

Depending on your location - Apple may offer Refurbished relatively new Desktop and Laptops at a reduced price ( Apple Reduced Price lol )

Have purchased several times and very happy with the quality and reliability of the units.

Food for thought

Welcome.

The units I have purchased as Refurbished even had a Brand New Battery with only 1 Battery Cycle..

You can, if desired, purchase Apple Care + at that time and the machine does come with the 90 Day Telepone Support and 1 Year Warranty.

Starting to sound like a Car Saleperson :-) lol but really trying to avoid unneeded cost during this Pandemic crunch.