All the Brexit argument seems to rest on a highly unlikely assumption, that trade with the EU would just continue as normal (but without all those pesky regulations) and the rest of the world would come rushing in offering us favourable trade deals.
The reality is that if we are going to continue trading with the EU at around current levels, we will have to comply with their regulations plus a few more - but of course we won't have any input in framing those regulations. We may eventually be able to establish good trading relationships with other major markets, but that will require time and hard work and there are no guarantees.
The Outers accuse the Treasury of scaremongering, but it would be extraordinary if Britain were able to make such a huge shift without major perturbations to the markets and currency.
So the 'Best Case' scenario for Brexit might be that we only suffer as short recession (maybe just two to three years) after which things settle down but we still have to comply to most EU laws - but will have more freedom to trade with other countries - if we can negotiate some good deals. We wouldn't have to accept EU migrants, but we would still need an awful lot of them, so numbers probably wouldn't change much (unless the recession was really bad which would admittedly cause a significant reduction)
