How to check custom object have no. of records by SOQL Query

If you want to check custom object have no. of records by SOQL Query
or their Record size then you need to make a list.

List mwList = [
   SELECT Id FROM Multi_Weather__c];
System.debug('mwList size: ' + mwList.size());

You Will get the no. of records in custom object for Example mwList size:2