When the user wants to create a new record, field with sequence should be read only and should show a message, that the number will be generated when the record is saved. What is the best approach to achieve it?
It is fairly simple. Make the field mandatory and assign it a DefaultValue which will be set to a string similar to <auto generated>.
Then when a new record is created by the user in the GUI it will show the temporary value. After saving it the state workflow will execute the sequential workflow which will assign a number from a number sequence. Immediately after saving the number will appear to the user instead of the temporary value.