Restrictionist arguments come in two flavors. One flavor is numbers-based — it is focused on the number of immigrants, rather than their specific characteristics.
The other flavor is characteristics-based — it is focused no the characteristics of immigrants that harm or threaten immigrant-receiving countries. These characteristics may include IQ deficit, dysfunctional immigrant culture, and skills mismatch. A slight variant on characteristics-based concerns is heterogeneity-related concerns.
Examples of numbers-based restrictionists include NumbersUSA, headed by Dr. Roy Beck. They state clearly on their website that their primary concern is with the numbers of immigrants, rather than the characteristics of these immigrants (see their “No” to Immigrant Bashing page). NumbersUSA is focused on issues of overpopulation and environment, so their numbers focus is appropriate.
The Center for Immigration Studies (billed as “low immigration, pro immigrant”), headed by Mark Krikorian, employs a mix of numbers-based and characteristics-based arguments, but with more focus on the numbers angle.
There are few examples of pure characteristics-based restrictionists — most characteristics-based restrictionists also employ numbers-based arguments to broaden their appeal. VDARE is an example that comes close to being mostly focused on characteristics-based restrictionism.