AC6 has around 35 missions per run once you remove the decision missions, which is comparable to AC2 which had around the same but AC3 had about 50 missions.
For me the main difference is two fold. In the older titles aside from the early missions the maps were generally longer both in size and the time it took for you the player to complete them. Older titles rewarded you for not using too much ammo or getting hit which for me incentivized playing slow then going fast on the replay run. (Also no repair kits haha so you really didn't want to lose
AP early because you never knew when you would be surprised by a rival
AC 
).
Couple that with AC6 being much faster than
Gen 2 or
Gen 3 and you complete missions faster in general. Some of the AC6 maps feel much larger than AC2 or AC3 but when you move quickly and boost recharges infinitely faster for most builds you get through them faster. It's a good thing
IMO but it does make me nostalgic for plodding through underground corridors instead of overboosting everywhere.
I think someone mentioned earlier that AC6 has more of an arcade feel with fast missions that don't waste your time and I think that's a pretty accurate assessment.