Yes, I believe one trading strategy is enough if a trader masters it fully, but that strategy won't work on all market conditions that's another fact and such a trader should understand that thing.
I know many traders who still follow Bollinger bands and Rsi strategy, personally I also somehow follow that strategy from a long time and so far it has given me good results.
And, there's group of trader who believe in buying the dip and holding it for sometime and make profit from the holding, I also follow that strategy and that one has also been really profitable for me.
Those two strategies can be combined into one strategy and they'll still work because when market is dumped then surely RSI and Bollinger Bands strategy also get fulfilled and combining those two results in even better profits if trader is emotionally strong.
You can have one strategy, but you can use many different tools in the analysis, and ultimately you should definitely adhere to a number of rules, in which case the trader will have a much better chance of achieving profit in trading.
It seems to me that more often we are talking about one strategy, because it is not so easy to thoroughly master several at once, maybe it is possible, but it should be the main activity for a person, and if it is like a hobby, one of the activities in addition to the main job, then it is better to focus on one strategy and become an expert in it, in which case it seems to me that achieving results will be more realistic.