The parameter for setting the number of mismatches in standalone blast
There is no direct option like that but you can set percentage identity filter with -perc_identity

However, this is only for reporting. BLAST will still perform all the alignments. That is why I suggested you to use bowtie. 

EDIT-1 (getting long for comment)

If you want a word type of search then use the -n option. But it always starts from the 5'end which is quite justified in case of miRNA because the seed region is in the 5'. 

Setting -n to 0 means no mismatch in seed region. -l option defines the seed length; I would advise that you use something like 10. -e refers to maximum mismatches in the non seed region but it is not the number of mismatches; it means maximum tolerated sum of phred score of mismatches (fasta files are assigned a default phred score of 40 at all residues. So, for 2 mismatches you have to mention -e 80).

EDIT-2

Yes. Increasing mismatch and gap penalties would make the search more stringent if you have already specified a cutoff on score.