100

100 solenoids were aggregated and made into one devices.

spec
Solenoid x 100(https://www.sparkfun.com/products/11015)
M5Stamp C3(I2C controller), self-designed PCB(16 solenoids assigned on 1 board)

sample source code.

#include <Adafruit_MCP23X17.h>
#define MCP_LEN 1
Adafruit_MCP23X17 mcp[8] = {Adafruit_MCP23X17(), Adafruit_MCP23X17 (),
                            Adafruit_MCP23X17(), Adafruit_MCP23X17 (),
                            Adafruit_MCP23X17(), Adafruit_MCP23X17 (),
                            Adafruit_MCP23X17(), Adafruit_MCP23X17 ()};

/* 
 *  10x10 solenoid x-y matrix position
 *            (9 9)
 *  |---------|  
 *  |         |
 *  |         |
 *  |         |
 *  |---------|
 *(0, 0)
 */
void updown(int x, int y, int d) {
  int s = ( x * 10 + y ) / 16 ;
  int p = ( x * 10 + y ) % 16 ;
  mcp[s].digitalWrite(p, HIGH);
  delay(d);
  mcp[s].digitalWrite(p, LOW);
  delay(d);
}

void testfunc() {
  for (int i = 0; i < MCP_LEN; i++) {
    for (int j = 0; j < 16; j++) {
      mcp[i].digitalWrite(j, HIGH);
      delay(500);
      mcp[i].digitalWrite(j, LOW);
      delay(500);
    }
  }
}

void setup() {
  Serial.begin(9600);
  Serial.println("Adafruit_MCP23X17 Test!");
  pinMode(6, INPUT);

  for (int i = 0; i < mcp_len; i++) {
    if (!mcp[i].begin_I2C(0x20+ i)) {
      Serial.print(i);
      Serial.println(" Error.");
    }
    delay(100);
    for (int j = 0; j < 16; j++) {
      mcp[i].pinMode(j, OUTPUT); 
      delay(40);
    }  
  }
  Serial.println("Looping...");
}

void loop() {
  testfunc();
}

4×4, circle type enclosure for test.

board designed.
IO Expander : MCP23017
MOSFET : 2SK4017
register: RK73B1JTTD103J 1608(0603)
diode : GS1010FL
connector : S2B-PH-K-S
solenoid : ZHO-0420S-05A4.5(5V)

Solenoid mounter 3D model is three parts.
Top is just an solenoids mounter, 2nd parts holds up to 100 solenoids and guides the cables, 3rd is guide to the output cables.