These are the different spellings for this surname in WikiTree:
San Miguel (51 profiles)
SanMiguel/Sanmiguel (30 profiles)
San Migel (4 profiles)
Let's see Rosa (SanMiguel-22)'s profile:
She was born in Texas, USA on Oct. 1885, she was the daughter of Antonio San Miguel and Thomasa Garcia.
I found a probable record of her christening:
Name: Rosa San Miguel
Christening Date: 11 Oct 1885
Christening Place: San Francisco De La Espada-catholic, San Antonio, Bexar, Texas
Father's Name: Antonio San Miguel
Mother's Name: Tomasa Valdez
Indexing Project (Batch) Number: C59032-1
System Origin: Texas-ODM
GS Film number: 25488
The book's index: https://familysearch.org/ark:/61903/3:1:3QSQ-G9NB-XVKC lists this record in the column on the left, 7th. row from the bottom:
San Miguel. Rosa. B. 249. very poorly legible.
The actual record is here: https://familysearch.org/ark:/61903/3:1:3QSQ-G9NB-XV45 (Right side; Almost bottom of the page). The marginalia of the record is: Rosa San miguel "civil" (Civil: Her parents got married by the low).
Her father is listed Anto San miguel in the record.
Even though the surname is written with no capital "M" in the record (Editor's capital "M" can be seen in record No.250: .... y d. Manuela Hernandez .....) the correct Spanish spelling is San Miguel as Vincent said.