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A pretty big indoor bush. I need more than 70 clones every 8 weeks so I keep 6 MOMs and only keep them for 3 months at a time and then the next 6 MOMs from her clones come into place and I repeat all year. My 6 plants are about 20" bushes.
If you need 70 at the same time, yeah, it'll need to be pretty big. I can take 15-20 off a bonsai every 2 weeks if I feed her properly, she's about 8" tall in 16oz of soil ... two of those would have you covered if weekly production does the job. Seach 'bonsai' and find the threads by DrBudGreenGenes and the repost of Oldtimer1 ... good info in both of those.
I have 5-6 ft. mother plants that when I clone them to death I can usually get a whole tray of clones(72). Other mothers I have can be half that size and still give me the same amount of clones. It all depends on growing style, and strain. The mother will also need lots of space to fill up while growing. If you can't make that happen, I would suggest multiple mothers of the same strain.
crusader is correct you can pull 1 or2 clones off each clone so you can get 70 or 140 ether grab them from you lower branches that where the best hormones are for your baby's or top each one if you like to keep them short or you have to have at least a 5ft mom mine is about 4ft and I can grab a easy 50 baby's