trigger counts on Opportunity(after insert,after update, after undelete, after delete) {
//clean list to add acc id too
LIST<ID> acctID = new LIST<ID>(); 
    //is the trigger context an insert
    if(trigger.isInsert){
        //loop through the opportunity id's in the new array   
        for(Opportunity o : trigger.new){
            //add the new opportunity account id to the list
            acctID.add(o.AccountId); 
            }
        }
     //is the trigger context an update - same as above except for update
    if(trigger.isUpdate){
        for(Opportunity o : trigger.old){
            acctID.add(o.AccountId);
            }
        }
    //new list for the acct that the opportunities are associated to
    LIST<Account> acct = [SELECT id FROM Account WHERE ID IN :acctID];
    //list of opportunities with the account as the parent id
    LIST<Opportunity> oppty = [SELECT id FROM Opportunity WHERE AccountId IN :acctID];
    //clean list for accounts to update
    LIST<Account> updateAcct = new LIST<Account>(); 
    //loop through accounts being updated
    for(Account a : acct){
        //get the size of the opportunity list
        Integer count = oppty.size(); 
        //if the count is not equal to 0
        if(count != 0){
            //assign the count to the count field on account
            a.count__c = count; 
            //add the record to the account list to update
            updateAcct.add(a); 
        }
        //update the account
        update a; 
    }
 
    
}
 

 Try this, that way the roll up will also react to updated opptys, as well as deletes and undeletes. 

--KC