There is only one single tempdb, but a database can be split up on many filegroups, and a filegroup can consist of several files. For tempdb, you typically only have a single filegroup, but it is recommended to have many files.
When there are multiple files in a filegroup, SQL Server will allocate space in these files in a round-robin fashion. That is, if you create a temp table and fill it up with lots of data, the first extent of eight pages will be on, say, file 2. Next extent will be on file 3 etc. If you have 12 files, the 13th extent will be next to appear in file 2. This presumes that all files are of the same size, which is also is highly recommended.
To this comes the log file. You can have multiple log files, but in difference to data files, there is little point in this, so typically you only have one log file. Since tempdb is in simple recovery, the transaction log for tempdb is frequently truncated, but if there is a long-running transaction, it can still grow and eventually eat up all available disk space.