Multi-monitor workstations could be necessary only if you need to watch several markets or trade several instruments (for example, stocks and options at the same time), but there is no need in such systems for the reatil trader. The best solution would be a high-quality single 22-24" monitor - it would be enough for almost all possible scenarious.
In fact, hardware is not such important for trading, as it deemed. The overall perfomance of the system, its speed, is much more important than the screen size. That is why many traders (mostly active daytraders) use gaming laptops. Additional monitor could be necessary if you would like to watch positions and make a research at the same time. For example, you can use one monitor to keep the platform opened while using second monitor to read the news or launch additional tools like Forex Tester (backtesting software) or to work with your trading journal. Most of the desktops and laptops allow to connect second monitor without additional modifications, so this would be the easiest way to create enough “screen space”. If you have no possibility to add second monitor, make sure your system works as fast, as possible, so that you could compensate the lack of space by switching from one window to another.