What is the default line spacing in Libre office writer?
This question makes no sense by itself. The answer is in the way you configure the styles (or how it is configured from retrieval of your OS default settings – it would have been good you mention it; you can do it editing your question, not answering).
The basic default is in Tools>Options, LibreOffice Writer>Basic Fonts (Western), assuming you’re writing a non-Asian language. There is also a similar option for Asian or “complex text layout” if enabled.
There you’ll find the font face and size for various primary paragraph styles. For short, Default controls Default Style which is the ancestor of all other styles. Heading:, … overrides Default: for their descendant styles.
The size will select the default line spacing according to the font properties.
If this does not fit your needs, you can “correct” this modifying Default Style in its Indents & Spacing tab. This will propagate to all other styles. This may not always be desirable. You can be more selective by playing with Text Body which is the “normal” style for text in a document. Default Style should never be used; it is reserved for setting document-wide defaults.
I intentionally do not explain in details how to configure styles to encourage you to read the user’s manual.
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The font metric defines, among other things, the “single line spacing” that separates the lines. Paragraph styles in a Writer document can modify that setting by increasing or even reducing the default value. For instance, the build-in default template in Writer uses a 1.15 multiplier for the font’s default in the “Body Text” paragraph style, and that means that the line spacing for that style will be a 15 % greater than what’s indicated in the font. You can of course change those defaults and even the set your own default template, but you’ll need to learn about the use of styles: that’s a topic too big for a short answer.