Google Social Security Risk of Dying table, or something similar. I did that a few years ago. It's interesting to look at hard numbers.
So, in your case OP:
You and spouse are both 28 years old. Consulting the copy of the table from a couple years back show this:
Of each 100k born, at 28 years old:
There are 97,642 men and 98,673 women still alive.
Over the next 10, 20 and 30 years, the odds of a 28 year old man dying are 1.52%, 4.29% and 10.47% respectively. (96,154, 93,457 and 87,419 still alive at 38, 48 and 58 years old, out of the 100k)
Over the next 10, 20 and 30 years, the odds of a 28 year old woman dying are 0.78%, 2.51% and 6.30% respectively. (97,907, 96,193, and 92,461 still alive at 38, 48 and 58 years old, out of the 100k)
So, OP, I believe you indicated you're the gal - you have an excellent chance of surviving 20 years as only 1 in 40 women who are 28 will die before reaching 48. Your hubs, he has a 1 in 23 chance of dying. Those are the odds, per Social Security.
You'll probably want to adjust those policy amounts (yours and his) to take into account your expected relative income levels, since that's what you're out to replace in the event one of you is to die. The one you anticipate to be the bigger earner over the term should have the bigger policy in proportion, since there is more to lose should their number get called versus the lower income earner.
You'll probably ALSO want to take the face policy amount and discount that by your estimate of inflation at the critical year during the term (e.g. what would be the worst year that one could die, taking into account expected asset accumulation to begin self insuring, etc). So, should someone die 12 years from now, when your soon to be had child is about 10-11 years old, that 500k (in today's value) policy will have substantially less purchasing power due to 2-3% / year inflation. Of course, hopefully you've accumulated assets by then to compensate (say in Jr's college fund), but, take those two effects into account in deciding what enough means.
It sucks to contemplate mortality. Even worse is to ignore it, and have your other half be the one 1 in 23 or 1 in 40 leaving you stuck holding the bag.