Old dynamis was a closed event that only one group of up to 64 members could take up in at once with a 72hr waiting period to re-enter. You would have to buy a glass for 500k then trade it to the marking which would allow you to make copies of another glass that you trade to members so they could freely enter.

The new dynamis which had just recently been changed for the older areas is one in which you only pay 50k straight fee and can enter once a day with it resetting on JP midnight. the new dynamis is open for all and not instanced with prespawned mobs and using popp items to spawn certain NMs and the boss. With the new dyna it's easier to get clears though they have added tougher NMs with more loot adding a new reason to actually do the event.

So basically dynamis has nothing to do with the way dungeons work at all a closer parallel would be to look at assault and MMM in terms of "restriction" to how many groups can do it at one. Hopefully with the fact that it is up to 8 member event
there will be a decent amount of instances to hopefully allow at least 20 groups to enter per server. or even more. But if it turns out to be like XI in the sense of BCNMS and such then there will only be 3 instances however with how guild leves were handled I feel as if dungeons will work in a similar manner. I'm pretty sure over 20 people can do the same exact guild leve and such but we'll find out in the coming weeks how restricitive the entry limit is for the dungeons.