Specific recommendations for per-library input mass for different enrichment project types can be found in the applicable myBaits manual.
Target capture necessarily requires subjecting your libraries to a bottleneck, wherein target molecules are captured and therefore enriched, and non-target molecules are therefore removed. To have sufficient unique molecules for good sequencing coverage of your targets, successful captures DEPEND on the input of sufficiently complex libraries.
For best results, it is recommended that only amplified (non-PCR-free) NGS libraries are used for target capture. This provides multiple copies of each starting template molecule, increasing the chance of each individual molecule getting enriched. However if you need more starting material to reach the recommended amount, it is generally preferable to generate more library from fresh genomic DNA or a new batch of indexed library, rather than through extra amplification. This is because while some amplification is good, over-amplification risks reducing the observable complexity of your libraries through the uneven action of PCR bias, as some molecules will become relatively more abundant while others become rare. This is also true for manipulating your libraries after capture: amplify your post-capture libraries the minimum number of cycles necessary to reach the molarity required by your sequencing facility.