For 100 LEDs in parallel you will also need 100 resistors, and a transistor (MOSFET) that can handle the total current for all the LEDs.
For instance, at 20mA drive current you would be dealing with 2A - you'd need a (minimum) 2A power supply and a MOSFET that can handle 2A.
Or you can think a little differently.
If you use a higher voltage power supply you can group your LEDs together into chains. For example with a 12V power supply you could (depending on the voltage of the LEDs) have up to 4 in a chain (red LEDs, 2V forward voltage, 2x4=8. White LEDs, 3V forward voltage, 3x4=12). Each chain then has a single resistor, and you're working with a quarter of the current (500mA) - just 25 chains of 20mA but at a higher voltage. So you can use a smaller MOSFET and you have only 25 resistors.
Another thing you could consider is to then further group those chains together. Maybe 5 groups of 5 chains, each with their own MOSFET. Then you switch just one group on at a time, but cycle through them very rapidly. Each group only then uses 100mA (5 chains at 20mA = 100mA), and you can use a really small MOSFET, and your power supply needs only be able to provide a couple of hundred mA (give it some headroom) and can be smaller and cheaper.
The only down-side of that method is you can get flicker which may be noticeable if the LEDs are moving. It also means your Arduino is doing more work to light all the LEDs.