You sure can. SFTP allows you to upload as many files at once as you want. There are no limitations on our side. But it’s worth noting that most FTP client applications (e.g. FileZilla) set a limit of possible uploads. So if you try to upload more than that limit, all additional files will be put into a queue. Upload will start once the first ones are done.
If you prefer to upload more files at once, you can change the settings of your FTP client application. Keep in mind though that the more files you upload at the same time the longer the upload of each file will take.