A general rule of thumb is that you have 3 board members at the seed (1 non-CEO founder, the CEO which is typically another founder, and the lead investor). So you have 1 seat available for investors, whereas you may take 5-20 checks. Not everyone is getting a seat.

At the A you usually expand to 5, adding the lead of the A round and an independent board member. Beyond that, it’s common for the earlier investors to get replaced on the board in future rounds and maintain observer rights.