The issue with growing asparagus is that you need to give it quite a lot of space. And since it provides very little ground cover it needs a lot of weeding. Though maybe you can black plastic the outer areas, and all of it during winter to keep down the weeds. Any standard book will tell you the crown spacing so work out how much space you can give it and then you can compute the number of crowns. I'd suggest that anything less than a bed about 8ft by 2ft6 wouldn't be worth the bother unless its just a novelty to give you a couple of snacks.

I really wouldn't put salt water on it as another poster suggested. It is not a halophyte, and doesn't grow in marshes either, it grows in well-drained places. I think this is a confusion with the custom of using seaweed as a mulch for it. Either that or he is confusing it with marsh samphire, which does grow in salt marshes, and has certian asparagus-like qualities.

Here's a growing guide. No doubt you can find more for second opinions. Growing Asparagus - How to Grow Asparagus from Allotment Vegetable Growing Advice and Guides