At 20mA, that's about 120 Ohms. The current gain depends on the collector current, but figure on 50. With a base current of 20mA, that's a collector current of way more than the 2N3904 can handle, so 20mA will saturate it thoroughly. You can probably get by with a lower base current. Look at a 2N3904 datasheet for gain at different collector ...

It is a little more powerful than the 2n3904. Most likely the current gain is less, and and maybe also the maximal voltages. Depending on your application that may be OK, which is quite likely if it is not too demanding. The correct thing would for you to check the numbers in the data sheet. 2N3904 데이터시트(무료), 2N3904 Datasheet, 2N3904 데이타시트, 2N3904 PDF, 2N3904 가격 구매, 2N3904 제품상세정보. 2N3904 쇼핑몰, 파는곳 ...