Depends what you are trying to do.

As Neilfws says, if you want to limit the number of hits reported you can use (from the NCBI BLAST+ help output):

 -num_descriptions <Integer, >=0>
   Number of database sequences to show one-line descriptions for
   Not applicable for outfmt > 4
   Default = `500'
    * Incompatible with:  max_target_seqs
 -num_alignments <Integer, >=0>
   Number of database sequences to show alignments for
   Default = `250'
    * Incompatible with:  max_target_seqs

These correspond to the '-v' and '-b' options in legacy NCBI BLAST:

  -v  Number of database sequences to show one-line descriptions for (V) [Integer]
    default = 500
  -b  Number of database sequence to show alignments for (B) [Integer]
    default = 250

The '-K' option in legacy NCBI BLAST:

  -K  Number of best hits from a region to keep. Off by default.
If used a value of 100 is recommended.  Very high values of -v or -b is also suggested [Integer]

Is slightly different and maps to the '-culling_limit' parameter in NCBI BLAST+:

 -culling_limit <Integer, >=0>
   If the query range of a hit is enveloped by that of at least this many
   higher-scoring hits, delete the hit
    * Incompatible with:  best_hit_overhang, best_hit_score_edge

You may also want to limit the number of matches reported per hit (i.e. limit the number of HSPs):

 -max_hsps <Integer, >=0>
   Set maximum number of HSPs per subject sequence to save (0 means no limit)
   Default = `0'

For more information about the NCBI BLAST+ command-line options see:

• NCBI BLAST+ usage/help output (run the relevant BLAST program with the '-help' option)
• BLAST Command Line Applications User Manual
• NCBI's BLAST help