Los Angeles has received numbers of immigrants in recent years.
Los Angeles has received a number of immigrants in recent years.
Los Angeles has received a number of immigrant in recent years.
Simply, 'immigrants' has to be in the plural because more than one came. You could use it as compund, in which case it would not take the plural:
Time- this would be plural when in meant 'on every occasion/'- All the times I met him,
Most (of the) Americans- always plural.