Do Daleks have gender? How do they reproduce? Cloning?
No, many stories, like The Power of the Daleks or Daleks in Manhatten indicate the Kaled mutant can be grown in a lab like environment, given the right materials. Add to that a specifically created Scientist Dalek and it shouldn't be too hard. The Progenitor from Victory of the Daleks clearly was designed like a 'seed ship' of sorts that creates Daleks specific for the needs of making a new colony.
This Parliament recovered the massive Dalek Army the Doctor buried under all that molten ice on Spidiron? So that's the idea behind this massive Dalek civilization, find the fallen, nurse them back to health until they can sic balls? I mean with time travel, even time scoops at work, they can totally freejack all the lost causes they left for dead the first time around.
Interesting notion, they could've salvaged what they could from past adventures and kept the sane ones and put the rest in the Asylum in a bid to nurse them back to health. Clearly the size of the population grew too much to handle/stability of the population was deemed to irrelevant to fix and was thus sealed off.
It's possible they raided past Dalek losses in a bid to gain raw materials for creating their new army and the Asylum became a bi-product. OR during the Time War the Daleks were scooped up from all time zones and those that couldn't fight were dumped in there.