• Are you asking for the number of GPIO pins available on the STM32F405RGT6?
• Or the count of serial interfaces (SPI, I²C, UART, etc.) provided on this board?

• The total number of general-purpose I/O (GPIO) pins broken out on the STM32F405RGT6?
• The number of enabled serial/peripheral interfaces (e.g., SPI, I²C, UART) on the board?
• Or something else (e.g., ADC channels, timers, PWM outputs)?

How do I start the PCB layout after completing the schematic?
Click the PCB tab. Your schematic components become footprints on the board. Define the board outline and arrange components based on the ratsnest. For more information checkout this Getting started tutorial