The standard Space Marine terminators box comes with 40mm bases. Many Space Marine character models also come with 40mm bases (e.g. Primaris Captain, Captain in Terminator Armor), though some like the Tech Marine have smaller ones.
My picture below is not entirely neat or to scale but should give you an idea. If you arrange terminators around the character in a perfect 7x7 grid then only the four corner models will not be wholly within 6". Also discounting the character himself leaves 44 terminators. You can almost certainly cram another two along each edge by placing them outside of the grid pattern, which would allow 52 in total. Deviating from the perfect grid may allow you to get a couple more in there somehow. However, this assumes that none of the models overlap their bases at all and that you have a large, open area to deploy into. In most cases there will be terrain in the way. It would also be almost impossible to move and retain this formation.
In practical terms, getting to 52 would require you to take at least 6 units of terminators. Some settings don't allow you to take the same datasheet in your army more than three times, so you might need a mix of regular terminators, assault terminators and relic terminators. You can't fit six elite slots into a regular detachment so you will need to pay extra command points in one way or another. Also, terminator costs vary depending on equipment but 52 of them are roughly going to cost a little over 2000 points on their own. This is to say nothing of the tactical folly of fielding an army with nothing but slow terminators. In other words, you can fit more terminators than it would every be practical to take.
Of course, the
40k rules almost never refer to 'models wholly within' a certain distance so I am not sure how any of this is relevant. Most aura-type rules just say 'units within' a certain distance, which would give you much more flexibility. In most cases you could easily arrange for two 10-man terminator squads to be within six inches of a character whilst still being able to move and avoid terrain. Three or more units may be possible if you position things very carefully and sacrifice a bit of movement.