How many legislators does it take to open arms talks in Geneva? The answer, it seems, is in the vicinity of 25 to 30, plus 10 to 20 sword bearers. Some 10 senators are scheduled to attend the opening of Soviet-American nuclear and space arms talks in Geneva on March 12. Perhaps 16 members of the House of Representatives will manage to make it too. With the senators will go four staff people; the congressmen will also have staff support. With the House delegation (but not, as members of it are quick to point out, the Senate delegation) there will also be some spouses. On the way back, some of the House people will stop off in Ireland to join the speaker, Tip O'Neill, who is headed there separately, for a parade.

Columnists Rowland Evans and Robert Novak reported the other day that there was much dark grumbling within the administration on the theme that the whole congressional role had turned into a circus, and the Kremlin might try to exploit it. Certainly a circus atmosphere is suggested by the piling up of numbers into the 40-plus range, the expected congestion on the briefing scene in Geneva and the unmistakable air of photo-opportunity that hangs over the proceedings. Not to speak of that stopoff in Ireland.

A more modestly sized delegation and one more evidently bent on business might have spared Congress a certain ridicule. Mae West once said that it was possible to have too much of a good thing and it was wonderful. But here it is not wonderful; it is absurd.

True, the extravaganza is for a worthy cause. Two months ago, the Senate announced, with President Reagan's favor, a plan to keep a rotating observer delegation in Geneva in order, as one of the observers, Sen. Daniel P. Moynihan, said, "to make a better connection between the negotiation process and the ratification procedure. There is a stack of unratified arms control treaties in the Senate already, and surely we don't need another." The undersecretary of state then invited Speaker O'Neill to send a group from the House to bolster congressional and bipartisan support. The House, controlled by Democrats, doesn't ratify treaties, but it controls weapons budgets.

The executive branch, any executive branch, is always nervous about Congress' butting in, and for a sound reason: Congress does butt in. We suppose it is worth a few days' discomfort at Geneva, however, if a two-way channel is cleared not only between Soviet and American negotiators but between the two American branches. It's just that there must have been a less farcical way.