When President Reagan made his television plea for the new tax bill Monday night, one of his selling points was the bill's extra unemployment insurance benefits for "states particularly hard hit by unemployment."

But the president, as usual, was being a bit conservative.

In fact, House-Senate conferees, eager to pick up support for the bill in both chambers, included every state in the special new benefit program, regardless of unemployment rate.

As a result, jobless workers in the 50 states plus the District of Columbia, Puerto Rico and the Virgin Islands would be eligible for an extra six to 10 weeks of unemployment benefits in addition to anything for which they might qualify under existing law.

With national unemployment at 9.8 percent, conferees made clear that they hardly felt any special justification was needed for helping people whose normal benefits had run out.

But they also made clear their hope that a provision for workers in a large number of states with plenty of House and Senate votes would win extra votes for the bill.

For a while, the conferees played with different formulas covering different blocks of states. The original House provision, pushed through the Ways and Means Committee by Rep. Harold E. Ford (D-Tenn.) months ago, would have covered 34 states. But changes later in unemployment rates threatened to knock two-thirds out, and Ford pressed the conferees to write a wider provision.

Three Democratic members of the California delegation, Fortney H. (Pete) Stark Jr., Don Edwards and Robert T. Matsui, reportedly told House Ways and Means Chairman Dan Rostenkowski (D-Ill.) that any formula excluding California might also exclude all or most of the 43 California votes in the House.

Inclusion of New York, which has a relatively low unemployment rate but 39 House votes, was a key consideration, especially since Reps. Charles B. Rangel (D-N.Y.) and Barber B. Conable Jr. (R-N.Y.) were members of the conference committee.

In the end, the conferees wound up including all the states. The cost, to be paid by the federal government, would be be $1.9 billion in fiscal 1983. The program would last through the end of next March.

The final provision was complicated.

In 38 jurisdictions (36 states, Puerto Rico and the Virgin Islands) which have, or have had since June 1, extra-high unemployment, the bill would grant long-term jobless workers up to 10 extra weeks of unemployment insurance benefits.

Under existing law, a worker is already eligible for 26 weeks of basic state benefits and in addition, if a state meets high-unemployment tests, another 13 weeks of "extended benefits" paid for half by the state and half by the federal government.

The 10 weeks provided by the tax bill would be in addition to any benefits under these two existing provisions. As a result, in some states with continuing high unemployment, some workers might get up to 49 weeks all told.

For states failing to meet an extra high-unemployment test, an extra eight weeks of benefits for a total of 34 would be provided by the bill, if the state has an "insured unemployment rate" of at least 3.5 percent. (The insured unemployment rate is the proportion of all covered workers who are receiving the initial 26 weeks of benefits. It is always lower than the total unemployment rate.)

In 13 jursidictions, which do not meet the 3.5 percent figure, six weeks of the special federal benefits would be available on top of the initial 26.

According to Labor Department projections, Maryland would be a 10-week state. Virginia and the District are six-week jurisdictions, although the District might become eligible for eight weeks of benefits in a few months.