Section 8.10.2 of IEEE 802.1D specifies the maximum diameter (The maximum bridge diameter of the Bridge LAN: The maximum number of Bridges between
any two points of attachment of end stations) as 7.
This is not, however, a set limit, it is a guidline derived when bridges were software devices and was based on the delay in transmitting BPDU's. In a modern switched environmnent it should be safe to go above this limit.
If, however, you wish to be in line with the IEEE standards you should conform to this.
