Sequence very often has to be used with Apply. Both of these functions are pretty strange and probably best avoided.

If you program the Wolfram Language like it's a functional programming language, you will never use these two functions and your code becomes much easier to read and test.

A variable number of manipulates... I guess the answer depends on how you have structured your code. Do you want the number of manipulates to depends dynamically on some kind of value?

n = 6

Manipulate[Evaluate@Table[u[i], {i, 1, n}], 
 Evaluate[Sequence @@ Table[{u[i], 0, 1}, {i, 1, n}]]]

Evaluate is needed to ensure that the arguments will evaluate. Manipulate holds its arguments. Sequence is needed to turn the list from Table into a sequence of statements.