Sow your onions now, your leeks can also be sown if you want them to be early or very large, but I usually hang fire until March.
For the onions, I sow into a seed tray in a 1x1inch grid pattern (in a cold greenhouse), and then prick out into 3" pots when the seedlings have straightened up.
My leeks are sown into a large plug tray and planted into 6" dibber holes when they are about pencil thickness.
As to quantities, work out how many of each you will need, and then sow twice as many. Plant out the ones which do best and compost the weaklings. :)