More than the limit of 'two' will certainly affect to the software itself as a human being its not possible to juggle over multiple applications and their multiple complexity of the problems. If you are managing near about 18 projects, then be sure to get everything veried by atleast 3 different teams. Again, you have to segregate separate time for suggested changes and some bugs (if any) to rectify the previous changes.
Think also about your health (Physical + Mental). Remember: Health is wealth, if your mind is happy, your software is happy, ultimately client is happy.
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