Compounds that have the same molecular formula but different chemical structures are called isomers. Depending on what types of differences there are between the structures, it is possible to classify isomers into various sub-types.
The Limit. You can follow up to 7,500 users before receiving the aforementioned error. Instagram invoked this limit to reduce spam in their system and keep users in compliance of their Community Guidelines. However, some users can follow more than 7,500 users if they did so before Instagram imposed the limit. Some Instagram users also claim there's an hourly or daily limit to the number of new followers you can acquire, but Instagram has not confirmed such restrictions.
On Twitter & Instagram I have estimated that the average interval time should be between 12 to 14 seconds. This therefore means that you should wait a minimum of 12 seconds between clicking follow or unfollow in order to not appear as an aggressive user.