To clear your doubt, if you measured your protein as x µg/ul and you are making the total amount as 50µg for your blotting in 20µl, this says it is 50µg in your whole 20µl. The only thing is concentration (x µg/µl) is diluted in 20µl but the final amount stays the same! (as Kenneth suggested).
To add some note, the amount seems bit higher for me (50µg), normally people use 10-20µg whole cell lysate for blotting. (you must increase if your protein of interest is less abundant!).
In contrary to Ariel, there is no gel with a protein capacity in µg! (you can theoretically load mg!, if you have such a high concentrated sample) I guess, she tried to mention as "µl". And I have no problem on loading 35µl volume in our biorad self-made mini gels. So this may be true only for the above said precast gels, doesnt mean that every gels have to be the same ;)