Position limits are defined on regulatory websites and may change periodically.  Some contracts also have near-term limit requirements (near-term position limits are applied to the side of the market for those contracts that are in the closest expiring month issued).  Traders are responsible for monitoring their positions as well as the defined limit quantities to ensure compliance.  The following information defines how position limits are calculated;

Option position limits are determined as follows:

• Bullish market direction -- long call & short put positions are aggregated and quantified in terms of equivalent shares of stock.

• Bearish market direction -- long put & short call positions are aggregated and quantified in terms of equivalent shares of stock.

The following examples, using the 25,000 option contract limit, illustrate the operation of position limits:

• Customer A, who is long 25,000 XYZ calls, may at the same time be short 25,000 XYZ calls, since long and short positions in the same class of options (i.e., in calls only or in puts only) are on opposite sides of the market and are not aggregated
• Customer B, who is long 25,000 XYZ calls, may at the same time be long 25,000 XYZ puts. Rule 4.11 does not require the aggregation of long call and long put (or short call and short put) positions, since they are on opposite sides of the market.
• Customer C, who is long 20,000 XYZ calls, may not at the same time be short more than 5,000 XYZ puts, since the 25,000 contract limit applies to the aggregate position of long calls and short puts in options covering the same underlying security. Similarly, if Customer C is also short 20,000 XYZ calls, he may not at the same time have a long position of more than 5,000 XYZ puts, since the 25,000 contract limit applies separately to the aggregation of short call and long put positions in options covering the same underlying security.

Notifications and restrictions:

IB will send notifications to customers regarding the option position limits at the following times:

• When a client exceeds 85% of the allowed limit IB will send a notification indicating this threshold has been exceeded
• When a client exceeds 95% of the allowed limit IB will place the account in closing only. This state will be maintained until the account falls below 85% of the allowed limit. New orders placed that would increase the position will be rejected.

Notes:

Position limits are set on the long and short side of the market separately (and not netted out).
Traders can use an underlying stock position as a "hedge" if they are over the limit on the long or short side (index options are reviewed on a case by case basis for purposes of determining which securities constitute a hedge).
Position information is aggregated across related accounts and accounts under common control.

Definition of related accounts:

IB considers related accounts to be any account in which an individual may be viewed as having influence over trading decisions. This includes, but is not limited to, aggregating an advisor sub-account with the advisor's account (and accounts under common control), joint accounts with individual accounts for the joint parties and organization accounts (where an individual is listed as an officer or trader) with other accounts for that individual.

Position limit exceptions:

Regulations permit clients to exceed a position limit if the positions under common control are hedged positions as specified by the relevant exchange. In general the hedges permitted by the US regulators that are recognized in the IB system include outright stock position hedges, conversions, reverse conversions and box spreads. Currently collar and reverse collar strategies are not supported hedges in the IB system. For more detail about the permissible hedge exemptions refer to the rules of the self regulatory organization for the relevant product.

OCC posts position limits defined by the option exchanges.   They can be found here.
http://www.optionsclearing.com/webapps/position-limits