What is a good max number of sensors for an Arduino Mega? Since it has like -- 2? -- ground connections, is it better to keep the number of sensors to a minimum amount?
It depends on what you want the Arduino to do with them. There is no (not much) penalty for having many sensors connected. Where it begins to matter is when you need to read at rates which, collectively, tax the speed of the processor to meet the total demand. Your project specs will really determine how many sensors make sense. The other place that sensor count might matter is if your project is to be battery powered, and the sensors draw some amount of current even when they're not being used. In a battery powered project, you'll want to be able to power-down everything that isn't being used at a given time, to maximize the system's overall run-time on battery.
The Mega, and I'm assuming you mean Mega 2560, has 50-some odd I/O pins available, so you have a lot of potential connectivity, and I think it's safe to say, the designers intended for them to be used.