Depends on ship's manuver.
As for T9 I use 4 limpets per beam. Good numbers are 3-4 per beam for big ships, 2-3 for mid-sized. This includes values like 2.5, meaning 5 for 2 beams.
Depends on limpet travel path - shorter the path, less limpets needed, faster the collection (in conjunction with the number of mining lasers).
Ideally you'd want to have no floating fragments waiting to be picked (which would imply bad positioning and/or not enough limpets), nor idling limpets (which would imply you need more mining lasers)
The Conda i posted above is sporting 13 collectors, 3 medium and 2 small efficient mining lasers (lets say the equivalent of 4 mediums) and with good positioning i got several idling limpets at any time during lasering an asteroid - that's why i said i could have used (at least) one more medium mining laser