When are the prefixes bis, tris etc. used in nomenclature of complexs?
For naming coordination compounds, the multiplicative prefixes ‘di’, ‘tri’, ‘tetra’, ‘penta’, etc., are used in the case of simple ligands, for example:
$\ce{[PtCl4]^2-}$  tetrachloridoplatinate(2−)
The alternative multiplicative prefixes ‘bis()’, ‘tris()’, ‘tetrakis()’, ‘pentakis()’, etc. are not only used for bidentate ligands. They are generally used with composite ligand names or in order to avoid ambiguity, for example:
$\ce{[Pt(PPh3)4]}$  tetrakis(triphenylphosphane)platinum(0)
The corresponding section in the current version of Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book) reads as follows:

IR-9.2.2.2 Number of ligands in a coordination entity
Two kinds of multiplicative prefix are available for indicating the number of each type of ligand within the name of the coordination entity (…).
(i) Prefixes di, tri, etc. are generally used with the names of simple ligands. Enclosing marks are not required.
(ii) Prefixes bis, tris, tetrakis, etc. are used with complex ligand names and in order to avoid ambiguity. Enclosing marks (…) must be placed around the multiplicand.

The use of multiplicative prefixes is also explained in the IUPAC Technical Report Brief guide to the nomenclature of inorganic chemistry. Pure Appl. Chem. 2015, 87(9–10), 1039–1049 as well as in the corresponding four-sided lift-out document, which is available as supplementary material.
Composite ligand names include ligand names which are substituted or which themselves contain multiplicative prefixes or locants.
For example, the normal prefix is used in ‘diammine’ $(\ce{(NH3)2})$, but the alternative multiplicative prefix is used in ‘bis(methylamine)’ $(\ce{(CH3NH2)2})$ to make a distinction from ‘dimethylamine’ $(\ce{(CH3)2NH})$.
Note that the use of the alternative multiplicative prefixes is not limited to coordination compounds. They are generally used for compositional names in inorganic chemistry when a simple multiplicative prefix may be misinterpreted. For example,
‘tris(iodide)’ is used for $\ce{(I- )3}$ (rather than ‘triiodide’, which is used for $\ce{I3-}$);
‘bis(phosphate)’ is used for $\ce{(PO4^3- )2}$ (rather than ‘diphosphate’, which is used for $\ce{P2O7^4-}$.