That limitation is not exactly biological. I'm pretty sure that alien mind (or artificial mind) would have pretty similar limitation. It could be "10 entities" instead of "5 entities", but the number of simultaneous entities in "processing model" would not be much higher.

The real reason for such limitation is that number of possible connections between objects is growing very quickly.

For example, 2 entities connect with each other by 1 connection.

3 entities connect by 3 connections.

4 entities connect by (4 * (4-1)) / 2 = 6 connections

5 entities -- 10 connections.

7 entities -- 21 connections.

10 entities -- 45 connections (4.5 times higher than for 5 entities).

Number of possible connections between entities defines complexity of the model.