You left unclear, what your exact requirements are. So the answer will be as general as your question. The SIM800L uses the Serial interface (UART). So the point is to connect to 100 different serial interfaces.
If it is ok to talk to each SIM800L sequentially (only one at a time), you can use a series of multiplexers. A multiplexer chip connects one pin to one of multiple pins (called channels here). Depending on the used chip, the channel can be selected in different ways. The standard way is with simple digital input pins, that construct a binary number. So for 8 channels you need 3 input pins (
2^3). Other ways are through communication interfaces like SPI or I2C.
Normally a multiplexer has only 8 channels, though you can also find once with more, but I don't think there are multiplexers with 100 channels. So you could next the multiplexers. If you put another multiplexer on every channel of the first multiplexer, you get
8*8 = 64 channels with 9 multiplexers in total. Add some more multiplexers on the next level and you will get to over 100 channels. Then you need 2 of these multiplexer trees to multiplex both serial lines (TX and RC). Since that are a lot of multiplexers and thus a lot of inputs to choose the correct channels, you could use multiplexers with SPI or I2C interface. The first would require 1 digital output pin of the Arduino per multiplexer as slave select pin, the latter would require either a multiplexer type, that can as many different I2C addresses as multiplexers in your system, or a bit more complex programming and also 1 pin per multiplexer to change the address of the chip to configure. How to use these multiplexers with Arduino depends on the used chip. You should refer to it's datasheet.
As you now see, connecting one single Arduino to 100 serial interfaces it a complex task. Depending on your actual requirements and goals in your "PoC project", it might be a better way to follow Majenkos advice and build it more modular with multiple Arduinos/microcontrollers.