For 5 min data, a few years worth of data is all you need.
They'll be so many 5 min candles that you'll have a decent sample size for analysis.
With very quick candle like 5 minutes, you don't need too much data like this. Because trading with 5-minute chart means you won't have to let your position opens too long. A longer you let it opens, with this short candle, a more risk you have with your position.
With Trading view, you can do back test and don't need to download data.
How to backtest a trading strategy.