Ther is only one pin GPIO21 marked wire SDA.
Can I put two or three wires on this pin if I call the two or more probes (for example an I2C capacitive probe) one after the other?
You can wire more than one I2C peripheral on the same I2C pins as long as each peripheral uses a different I2C address.
Alternatively, you can also set up other ESP32 pins to act as I2C pins.
We recommend reading this guide that covers those topics: https://randomnerdtutorials.com/esp32-i2c-communication-arduino-ide/
I hope this answers your question.
As Sara mentions you can use multiple I2C peripherals on the same bus (Pins) so long as they have different addresses. Some I2C boards have solder pads or jumpers so this is easy. Other boards, like displays, do not have these. In this case you can use an I2C multiplexer – https://tronixstuff.com/2019/10/29/tutorial-using-the-tca9548a-1-to-8-i2c-multiplexer-breakout-with-arduino/