qaqcmpm,
How can you be reviewing RT films if you are not familiar with the code ?
If you are passing judgement on someone or something you must be in possesion of all the facts.
Get the code and read it.
The answer to your first question is in the clause following the one you quoted.

6.1.2.3
One spot radiograph shall be taken in the first 3 m
(10 ft) of completed horizontal butt joint of the same type and
thickness (based on the thickness of the thinner plate at the
joint) without regard to the number of welders or welding
operators. Thereafter, one radiograph shall be taken in each
additional 60 m (200 ft) (approximately) and any remaining
major fraction of horizontal joint of the same type and thickness.
These radiographs are in addition to the radiographs of
junctions of vertical joints required by item c of 6.1.2.2 (see
Figure 6-1).

Second question took even less time to locate.
Try it yourself, go to 5.3.3 Examination and Testing of Tank Bottoms

I/we do not might giving assistance on this forum but you must make an effort to find the answers yourself, not jump on here and get us to do the work for you.
Sorry for the rant.
Regards,
Shane

The tank has already been built and you are reviewing the film?
The contractor that built the tank should have provided a shot sheet for the tank. You check it against what 650 requires.
It it not hard to calculate how many shot to take. D X pi (3.14) will give you the footage.
Guys with less than a high school education can calculate these.