You can top your plants indefinitely, if what you mean is topping for two or 4 tops, then letting them grow out and topping again on each of those for 4-16 or whatever, and then topping those tops etc.
HOWEVER if you do this you will either get scraggly growth and end up with bud that might as well have come from branches (however it helps with the light) or you will need A LOT of veg time. Logic kindof takes hold here. If, say, a plant with 1 top has enough root system to support growth of 1 inch and 2 nodes a day, then the same root system will take 2 days to do the same for 2 tops, more or less. In addition to this, topping slows the growth of the plant further until the wound has healed, although this is more noticeable in the folage and stems than the root ball development.
Some plants seem to be able to do this if you have the time to veg, have grown a plant from some bagseed to about 12" with 32 solid tops, but it has taken over 3 months. Essentially ruined another plant by doing the same topping to it, basically just a mass of branches now with no real structure.
Having done this, the way i currently top my plants is:
FROM SEED: let the seed grow to about its 8th node. Trim off all the growth below the 4th node, and then cut above the 5th node as close as you can without damaging any of the leaves, stems or branches at the 5th node. This will give you 4 strong tops on a plant in about 8 weeks total from seed.
From Clone: Take clones at least 4-5 nodes tall. Use the bottom node for your prefered cloning method, so you now have 3-4 nodes above the ground. Trim all the fan leaves on the plant, except the very newest set. This slows the clone growth while they root, and is a really easy marker
Once the clone is established, start with the first node that has trimmed fan leaves, and remove all the growth from the stem below that. This should be done no sooner than when the clone has grown 3 new nodes, but no later than when it has grown 5 or the plant will have wasted a lot of energy on what is essentially useless growth. Then top your plant 2 nodes above the cleared space for 4 tops.
With either of these methods, it should take arround 4-10 weeks to get a plant to have 4 solid tops. I know 8 weeks of veg seems like a lot, but if you have 1 space for flowering and 1 for veg/clones/mums, you can take the clones when you put the last set of cuttings into flower, and have your full 8 weeks to veg out to 4 tops.
For 2 tops, cut off the 5th node as well. You can probably do this in about 4 weeks from seed.
Hope this helps