I'm in the middle of a project and I've recently bought a 26,000mAh portable charger with 4 USB ports on which can each deliver 5V and 4.8A max. In my project, I'm using an amplifier which needs an input of 12V 2A. What can I use to get 12V at 2A from the USB ports on the portable charger?
5V * 4.8A = 24 Watts.
12V * 2A = 24 Watts
Since no step-up regulator or boost converter can do 100% efficient, you can't do this. Even assuming 90% instead of the typical 80%, that's 24 W * 1.1 = 26.4 W / 5 V = 5.3 Amps in. Half an Amp more than you can provide.
Now if your 12V device can run at a lower voltage or a lower current or both, it's possible. Use an ammeter with the amplifier at full strength to see what the peak current drawn actually is.
Consider getting a smaller or more power efficient amplifier, or a power bank that's designed for 12V and 5V out.
You cannot save money by buying a finished equipment that does not fit your direct power requirement, because the cost of developing of additional converter.
For your power level (12V 2A) I would consider something like a portable jump-start device used in car industry, like this one: