Extremly Customizable Site

Ok the website I am creating has to have the ability to be able to have unlimited amount of users. I know with limitation such as hardware right off the top that cant "just" happen but code wise it is possible. My main question though is how can I store large amount of information specfic from the user in a database. I could create a column but at times I will have columns that doesnt have a set value in it and I dont need to use. Such as I might have the option to store 20 contact numbers but the user only specfies 5. I dont want my query to check all 20 columns. I was thinking maybe having those kind of columns in a seperate table. But having ALL my users use that one table for storing contact numbers would be pretty slow if I say have 30,000,000 users and I wouldnt use an index cause something like that could change ALOT which would cause problems with updating the index. So I was curious stuff like that would it be that bad to store information like that in a seperate text file? Or is their a better way? All this information could be possible be read at any given moment once or alot more (other users can access other users pages). And having so much specific information and then could lead to some of the information being in an array like setting when I might have the ability to name all the jobs you have been in and some people can have 3 jobs and others have 20. So what is the best way to store alot of customer specific information for a whole lot of people