I *think* the rule of thumb is one TMU per 80 extensions, but I won't swear to it......with that said, we've had tone sender "always on" (parnum=56,parval=15)since we went to BC6 back in 1990 and have never had a problem. We're currently at BC13 with a mix of TMU's in some lims and TSU/TRU's in other LIMs.
Your mileage may vary.......
Dave Strang