You can focus on completing tasks for multiple projects, but I would advise you to create multiple accounts to receive more rewards. If you only concentrate on one project, and it doesn't distribute an Airdrop to users, you may feel disappointed and lose trust in Airdrop events, missing out on many other opportunities. In reality, tasks for projects with Airdrop programs can be quite challenging for newcomers. It's not as easy as before because there are many people cheating by using multiple accounts to earn more rewards. This also reduces the effectiveness of the project's marketing efforts.
If actually you acknowledge that the testing jobs are not always easy and are sometimes complicated for newbies and you also encourage that one should have it more than one. Maybe have to join many testing groups in order to earn more rewards. This simply means that the person will have to take it as a full time job because it will even be more complicated one you are handling such a task with more than 5 projects.
This is actually what gives opportunities for hack because you will need to create many wallets and connect them to different places you may not actually successfully trace when problem arises.
Before anyone will go into such a job, they should verify if it is still worth it at this stage of cryptocurrency evolution.