Want to know the official way of figuring out how many visas would be available for a specific month, in this case May 2013? FYI - The Philippines is second only to Mexico in terms of visa applicants waiting for their priority dates to be current.
As an overview, there is a worldwide limit of 226,000 for Family-sponsored categories, usually called Family Prefernce groups. Included are the petitons of US Citizens for their over-21 unmarried or married sons, daughters, siblings. The annual limit for employment-based is 140,000 and there are more Indians and Chinese than there are Filipino applicants. For 2013, the yearly limit is 158,000.
Then there is a per-country limit of 25,620. Since the demand (the number of visa applicants) exceed that of the visa allocation yearly, the waiting line for certain Family-based categories grow longer and therefore, backlogged. If the unused visas by some countries that do not use up their allocation are re-apportioned, the wait should be shorter. This in effect is what the bill H.R. 633 intends to address.
If the US Congress gets its act together and pass bills that would significantly add more visas in the Family-sponsored and eliminate the quota for Employment-based, then the waiting line will be shorter. One such bill based on legislative records is H.R. 633, former H.R. 3012, Which was Reintroduced in the House Referred to the House Judiciary Committee for Action
The links below are for the demand data - and immigrant visa waiting list - which are the main factors determining how fast - or slow - the March 2013 visa bulletin would be in the Employment and Family-based categories