[SalesForce] How to know what are the limits for Shield history tracking in salesforce

Create an object called AccountHistoryTracking and fields like APIName__c,OldValue__c,NewValue__c then create a fieldset on Account called'HistoryTracking' with the fields you want to track on Account and then write below code to track history on account.This way you don't need to worry about history tracking limit per object.

trigger AccountHistoryTracker on Account (after update) {

    final List<Schema.FieldSetMember> trackedFields = 
        SObjectType.Account.FieldSets.HistoryTracking.getFields();

    if (trackedFields.isEmpty()) return;

    final List<AccountHistoryTracking__c> fieldChanges = 
        new List<AccountHistoryTracking__c>();

    if(!trigger.isUpdate)
        return;

    for (Account newAccount : trigger.new) {

        final Account oldAccount = trigger.oldmap.get(newAccount.Id);

        for (Schema.FieldSetMember fsm : trackedFields) {

            String fieldName  = fsm.getFieldPath();
            String fieldLabel = fsm.getLabel();

            if (newAccount.get(fieldName) == oldAccount.get(fieldName))
                continue;

            String oldValue = String.valueOf(oldAccount.get(fieldName));
            String newValue = String.valueOf(newAccount.get(fieldName));

            if (oldValue != null && oldValue.length()>255) 
                oldValue = oldValue.substring(0,255);

            if (newValue != null && newValue.length()>255) 
                newValue = newValue.substring(0,255); 

            final AccountHistoryTracking__c accountHistory = 
                new AccountHistoryTracking__c();

            accountHistory.name         = fieldLabel;
            accountHistory.apiName__c   = fieldName;
            accountHistory.User__c      = newAccount.Id;
            accountHistory.ChangedBy__c = UserInfo.getUserId();
            accountHistory.OldValue__c  = oldValue;
            accountHistory.NewValue__c  = newValue;

            fieldChanges.add(accountHistory);
        }
    }

    if (!fieldChanges.isEmpty()) {
        insert fieldChanges;
    }
}

You can track upto 20 standard and custom fields per custom object.

To track field history for custom objects:

1. From Setup, click Create | Objects.

2. Click Edit next to the name of the custom object.

3. Select the Track Field History checkbox.

4. Click Save.

5. Click Set History Tracking in the Custom Fields & Relationships section. This section allows you to set a custom object’s history for both standard and custom fields.

6. Choose the fields you want tracked. You can select up to 20 standard and custom fields per object. You can’t track: Formula, roll-up summary, or auto-number fields Created By and Last Modified By

7. Click Save.Salesforce tracks history from this date and time forward. Changes made prior to this date and time are not included.

Look into this What is the best workaround for the 20 field history tracking cap?