After Sarria, several Camino routes converge, many pilgrims start their pilgrimages here, and the flow towards Santiago is logically much heavier. You are well-advised to make onward reservations, unless you are willing to take your chances on your first-choice of lodging being "completo" each evening.
You see, everyone else is going the same way as you, and everyone else wants what you do, a bed for the night. The good news is that it is only five or six days from Sarria to Santiago.
In your situation, one tactic that can work is finding someone who can speak Spanish, and who can use YOUR mobile to call ahead for reservations. The guide books have phone numbers for private albergues and hostals. if you do not have a mobile, someone else does, or the place you are staying may have a "land-line." If you have a guide book, consider a private albergue or hostal slightly before or slightly after you day's destination. I personally prefer slightly after, so I have less to walk the next day, in case it is wet, windy, and raw.
Absent having a walking companion who can help you, ask the hospitalero or hospitalera at your present lodgings (assuming they speak at least some English) to use your mobile (or theirs) to call ahead to book you a room. I have done this using gestures, a guidebook (pointing to the property or writing the name and number on a slip of paper), and my mobile. On some occasions, the person making the call for me had to call two or three places, but it still worked.
I have done this in my early Caminos, before I learned about Google Translate and Booking.com. IT WORKS!
Having said that, if you make a verbal booking this way, SHOW UP TIMELY! Usually the onward lodging will hold your room until about 1600. After that, you are likely to be dumped from the queue. Please, do act responsibly and with consideration and get there. If your plans change, DO find a way to contact the lodging and cancel the reservation so another pilgrim might have a place to sleep that night.
This will NOT work for public albergues. It usually only works for private albergues and hostals, on up...
I hope this helps.