I've heard that it is difficult or impossible to use budget reconciliation more than once or a few times per fiscal year, but are there hard rules that limit the frequency of its use to pass Senate legislature?
There are rules, but they depend in part on interpretation:
These are the so-called Byrd rules. A measure can't use reconcilliation:
- If it does not produce a change in outlays or revenues;
- If it produces an outlay increase or revenue decrease when the instructed committee is not in compliance with its instructions;
- If it is outside the jurisdiction of the committee that submitted the title or provision for inclusion in the reconciliation measure;
- If it produces a change in outlays or revenues which is merely incidental to the non-budgetary components of the provision;
- If it would increase the deficit for a fiscal year beyond those covered by the reconciliation measure (usually a period of ten years); or
- If it recommends changes in Social Security.
(quoted from wikipedia)
These rules can apply to a complete bill, or to individual clauses or parts of it. The interpretation of these rules is done by the Presiding Officer, acting (by convention) under the advice of the Senate Parliamentarian (an officer of the Senate appointed to give impartial advice on the interpretation of the Senate's rules) The presiding officer can be overruled by a vote of the Senate, but that vote is itself subject to cloture, unless various nuclear options are invoked.
In practice, finance bills are long and complex because changing the federal budget is complex. Changes in one area of spending have consequences in other areas of spending, and so practically it is impossible to construct several genuine budgetary measures in one year. So the real limit is not on the frequency of reconciliation, but on the types of measures that reconciliation can be used for. This effectively limits the number to only once or twice a year.
Under the Senates interpretation of the Congressional Budget Act, a budget reconciliation can only produce at most one bill on each of three subjects: tax raising, spending, and raising the debt limit. However as tax raising and spending are intimately linked, in practice, there will only be a single "tax and spend" bill, and perhaps a second "debt ceiling" bill. This limits the reconciliation process to being used at most twice in one year. However in the first year of a Presidential term, there is the opportunity for two budgets: one for the current year and one for next year.