You also have a few going to/from The Globe at various points of the day. Presumably this is to cater for the start/end of driver shifts. I reckon you're right, it could be as high as 7 or 8!
Morning peak
0600 ex Bath does 0700 ex Bristol arr Bath 0752
0630 ex Bath does 0730 ex Bristol arr Bath 0822
0646 ex Globe does 0738 ex Bristol arr Hicks Gate 0800 for 0802 dep
0701 ex Globe does 0753 ex Bristol arr Hicks Gate 0815 for 0817 dep
0716 ex Globe does 0808 ex Bristol arr Hicks Gate 0830 for 0832 dep
0731 ex Globe does 0823 ex Bristol arr Hicks Gate 0845 for 0847 dep
So I think it's 4 buses shuttling during the day with a couple extra at peak. Haven't worked out the evening but it looks a similar pattern. You could make the argument that the 0530 ex Bath won't be back til 0722 and so can't do the 0731 ex Globe so that could be 7 if you're stretching it.
36815 worked the 1308 (I think) but don't know if that was working for Bath or if it was the bus station spare or if it's a scheduled run (by Hengrove?) so a Bath driver can have a break!