Da un lato potresti usare sp_executesql:
exec sp_executesql N'select @rowcount=count(*) from anytable',
N'@rowcount int output', @rowcount output;
D'altra parte potresti usare una tabella temporanea:
declare @result table ([rowcount] int);
insert into @result ([rowcount])
exec (N'select count(*) from anytable');
declare @rowcount int = (select top (1) [rowcount] from @result);