Do you have to buy at least a quantity of 100 shares to equal 1 option ?

Question

does 1 option always equal 100 shares of the underlying stock ?

Answer 1

No, each contract represents 100 shares of stock.  You can't purchase the option on say 50 shares.

There's some exceptions, when the stock has undergone a split/consolidation or a merger, in which case you might have a different number of shares or another stock's shares.

Answer 2

Assuming you are talking about exchange traded options, traded on an exchange in the United States, every option contract when it starts trading is for 100 shares. However, there are times that an option is adjusted and after adjusting may represent less than 100 shares. For example, when JP Morgan took over Bear Stearns the options that were options for 100 shares of Bear Stearns stock became options for (A) 21 shares of JPM Common Stock, plus (B) $32.35 cash in lieu of 0.753 fractional share of JPM Common Stock.

For the details see

http://www.cboe.com/publish/TTStockSM/08...

It is also, in theory, possible to write up an offer on the over the counter market for an option that has whatever specifications you want. Given the high costs involved in such trading it would probably be impractical for less than 100 shares.