Best way to show items, whether use DialogueBox or UserControl?

It depends on what the forms are doing: if they are all doing similar things (say, looking a details of different people) then the same form makes sense.
If they are doing different things (say, one looks at peoples employment history, another edits current address and contact details) then different forms.
Generally, if you have to ask the question, you need different forms: keep them separate if the data is separate. Think about it: If one form has only 1 TextBox and two buttons, do you want it sharing the same screen real estate as a form containing ten TextBoxes, two drop down lists, and a DataGridView?

Many forms are certainly bad.

Use all in one form if you can. Is it UserControl or not — does not matter; you can do different designs with different techniques. Now, we're talking designs. You can use, let's say, 3 overlapping panels; on button click show one (discriminated by which button is pressed) and hide two. Also, use: tab interfaces, expanding panels, dockable interfaces, master/detail controlled by list or tree selection, etc. Most important: don't use MDI.

—SA