If else statement with a value that is part of a continuous character in R

My dataframe (df) contains a list of values which are labelled following a format of 'Month' 'Name of Site' and 'Camera No.'. I.e., if my value is 'DECBUTCAM27' then Dec-December, BUT-Name of Site and CAM27-Camera No.

I have 100 such values with 19 different site names.

I want to write an If else code such that only the site names are recognised and a corresponding number is added.

My initial idea was to add the corresponding number for all the 100 values, but since if else does not work beyond 50 values I couldnt use that option.

This is what I had written for the option that i had tried:

    df <- df2 %>% mutate(Site_ID =
ifelse (CT_Name == 'DECBUTCAM27', "1",
ifelse (CT_Name == 'DECBUTCAM28', "1",
ifelse (CT_Name == 'DECI2NCAM01', "2",
ifelse (CT_Name == 'DECI2NCAM07', "2",
ifelse (CT_Name == 'DECI5CAM39', "3",
ifelse (CT_Name == 'DECI5CAM40', "3","NoVal"))))))) 

I am looking for a code such that only the sites i.e., 'BUT', 'I2N' and 'I5' would be recognised and a corresponding number is added.

Any help would be greatly appreciated.