how to count polymorphic snps in vcf file

Question

[26bhagwat]

I have vcf file of 1000 samples. I want to use 500 sample for analysis..When I takeout 500 sample i want to check below

1. Monomorphic snps
2. remove monomorphic snps
3. after this I remove snps at maf0.1 but after this i got all monomorphic snps.. Please let me correct commands for each step. Maybe I am using something wrong process.

[Pierre Lindenbaum]

I don't understand. snps are by definition polymorphic.