There are a lot of futures contracts, and there's always a trade somewhere. Should a trader try to trade everything, or stick to trading one or two contracts they know well?
Whether you trade one or multiple futures contracts will depend a lot on the time frame you trade on. If you're a day trader, trading one to three different contracts will typically provide enough opportunities each day. This is especially true if trading off a tick, 1-minute or 5-minute chart.
The longer the time frame you trade on, the longer trades last and the more time you have to look for trades in other futures markets/contracts.
Trading an hourly, 4-hour or daily chart means trades could last several hours to days or weeks. In this case, trading multiple contracts is encouraged since there could be long periods (months even) where there isn't a single high quality trade setup in your preferred futures contract.
No matter what contract you trade though, know it well. Don't take random trades in markets you haven't researched or practiced trading in a demo account. Each futures contract may have different tick values, margin requirements, expiry dates, tendencies and news that affects it. Be aware of all these factors before trading. Knowing this information allows you to place informed entries, stop loss orders and target prices.
If you're a day trader, select one to three futures contracts and trade those. One of the most popular day trading markets is the E-mini S&P 500 (ES) contract. Another is Light Sweet Crude (CL). Trading one, both or another liquid futures contract will typically provide day traders with ample day trading opportunities.
If trading on a longer time frame, compile a list of futures contracts you are allowed to trade. Include that list in your trading plan and only trade contracts which are on the list. You should know about each contract on the list, including the news that affects it, tick values (so you can accurately calculate position size), margin requirements and any other information which will affect how you trade the contract.
Some traders trade one contract, while others trade multiple contracts. There is no right or wrong answer, but no matter which you choose, make sure you have researched and practiced your methods with each contract before risking real capital.