Creating subset is really simple. Let's say you have a samweb definition you are particularly interested in, like the anatrees for mcc8 bnb nu events (prodgenie_bnb_nu_uboone_mcc8_ana).
Then you can just create a subset of definition with the option with limit 1000

Like this:

samweb create-definition sdporzio_prodgenie_bnb_nu_uboone_mcc8_ana_50k defname:prodgenie_bnb_nu_uboone_mcc8_ana with limit 1000

You will have to replace sdporzio_prodgenie_bnb_nu_uboone_mcc8_ana_50k with the different name you want to give to your definition.