If your design needs memory, you have the following options:
The simplest way to store data is to use registers. It’s also very flexible: you can use as many as you want and you can read / write from them at any time. The downside is that they are not very efficient in terms of area.
As a rule of thumb, you can fit about 320 DFFs (40 bytes of memory) in a single tile. Since your design will also need some logic, you will probably be able to fit less than that.
Here are a few examples of projects that use registers as memory:
The DFF RAM compiler creates a dense array of DFFs that can be used as memory. It’s more efficient than using registers, but it’s also less flexible: you can only read / write one address at a time, and there are only a few sizes available.
For Tiny Tapeout, you can use the RAM32 macro, which is 128 bytes arranged as 32x32 bits (32 words of 32 bits each) with a single read/write port (1RW). This macro uses an area of 401 x 136 um, which fits in 3x2 tiles and uses about 54% of the area.
Including DFF RAM in your design is a bit more complicated than using registers, as connecting the power and ground pins is not trivial. You can use the DFF RAM example project as a starting point.
The RP2040 on the demo board can be configured to provide 64 kbytes of RAM to the chip over SPI thanks to the spi-ram-emu project by Mike Bell. It simulates the 23LC512 SPI RAM chip. This is a good option if you need a lot of memory, but it’s also the slowest option.