If jumping right up to 100 per day sounds too extreme, try challenging yourself more slowly by doing them EVERY day but adding just one more each day. Your body slowly builds strength and endurance and you can stop increasing when you reach your goal. And if you want to go higher in the future, just add one more that day! Good health!
Count them once they start hurting a bit is a great trick rather than counting every single one. Then do some more after a short rest. Try different exercises too and definitely planks you'll quickly get amazing abs.