x1 (set, group, structure, category, class, etc.) has cardinality less than or equal to aleph-null; x1 is mathematically countable (including the option of being finite).
Purposefully designed as a zi'evla. x1 is finite or countably infinite; there exists a bijection between (the set underlying) x1 and a (not necessarily proper) subset of the set of all of the natural numbers. The empty set is finite. For "uncountable"/"uncountably infinite", use "to'e (zei) kancuka'e". See also: cimni / tolcimni.