Generally you only get 5k base elite status qualifying miles w/ DL Amex Optima PLATINUM Skymiles signup/enrollment. Also w/ Skymiles Optima PLATINUM, you could get another 10k NON-status miles for signup/enrollment AND in addition with PLATINUM only, you get 10k base status qualifying miles for every $25k/yr that you spend.

Other "usual" past DL signup bonuses:

DL Skymiles Classic- 5k NON status mi
DL Skymiles Gold- 10k NON- status mi