If you have purchased the premium version of SoundSeeder, the number of connectable speakers is not limited. (See Free version vs. Premium version)
With a fast Wi-Fi connection (IEEE 802.11g+) you should be able to connect 16 Speakers (and more) without any problems.
Each Speaker requires a constant Wi-Fi traffic of about 1.5Mbit/s (0.75Mbit/s in mono mode).
Posted in: 3. Connectivity