How many Ekadashi are there in a year?
Not only Ekadashi but also all tithis (other than Purnima (full moon day) and Amavasya (no moon day) which occur only once in a month) come twice in a month i.e one in Krishna Paksha and one in Shukla Paksha.
So there are 24 Ekadashis in a year.
Note: The tithis are not same as dates which change daily after 12 o'clock in the night. There is no fix number of hours in tithis. Also there is a concept of Adhik Maas (extra month) which occurs about every 32.5 months to keep the lunar (a lunar year contains only 354.36 days) and solar calendars aligned.
Here is the list of 24 Ekadashi fasting days.
Note: Sometimes same tithi is listed on two consecutive days but that doesn't mean there are more than 24 Ekadashis. This is due to different sects' belief.
How many days after one another Ekadashi comes? and why?
Generally 15 days but not necessarily. As mentioned above Ekadashi re-occurs after 15 tithis but there is no fix duration for a tithi.
Does each one different from each other? why is it different?
Each one is associated with some story of Lord Vishnu. For example, DevShayani Ekadashi and DevUthani Ekadashi are completely different.
Does the sequence of Ekadashi remain same for each year? Why do they come in the same sequence?
Like we have Jan, Feb... similarly we have Chaitra, Vaisakha...
So sequence remains same.
Is there any Ekadashi, which is most important hence most people follow it?
This depends on opinion, belief and geographical location. In Narada Purana:
Sanatkumar says: "O Narada! A fast on Ekadashi (eleventh day) in both the phases of a month pleases Sri Hari and fulfills all the desires".
In my geographical location, few people give more importance to Nirjala, DevShayani, Parivartini (Parsva) and DevUthani Ekadashis.
Difference between Purnimanta and Amanta
Story of king Ambarīṣa : SB 9.4 and SB 9.5