indexing - How to handle database table with around 400 million records? -

i have 1 table 400 million records 5 columns. statsid (primary ke), date,time,eventtype,pubid(clustered index)

when run query in microsoft management studio "select count(1) table name" taking around 1 hour after tried rebuild index throwing error "collection modified; enumeration operation may not execute" please me why taking time execute simple select query.