Generally traffic from slave machines has a minimal impact on the master. The presence of more slaves generally accompanies an increase in INSERT/UPDATE/DELETE operations on the master as traffic increases. The capacity of a given master depends on its configuration and capabilities, but I can say that I have seen configurations with dozens of slaves to a single master. Those looking to decrease load on the master can use a tiered approach, with some slaves feeding others.
This was first published in May 2006