I would probably max that. 10% discount is nice. Often times there will be a holding period and you'll be given 10% off the lower of prices at either the beginning or end of the holding period. In that case it has the potential to be much more than a 10% discount, but worst case scenario it's still a 10% discount. You'd have to check your specific plan for the actual details. You'll be buying at least $22,222.22 worth of stock for $20k.
That discount will count as regular income when you sell it. If you purchase $100 worth of stock, and you pay $90, that $90 is taken out of your paycheck and is included with your regular income and you pay tax on that income the year you earned it. When you sell that $100 stock the $90 you already paid is accounted for, but the additional $10 will be realized and counted as regular earned income (and taxed as regular earned income - not as capital gains) from your company in the year you sell it.
EDIT: Forgot to say I would max it, but I would also sell each chunk of stock asap. This way you are only investing $20k, and holding $20k max in your company stock, and constantly churning that initial investment as you sell off the matured stock to rebuy new discounted stock.