Try this
If( Begins([Company Name],"a") OR Begins([Company Name],"b") OR Begins([Company Name],"c") OR Begins([Company Name],"d"), "A-D Clients",
Begins([Company Name],"e") OR Begins([Company Name],"f") OR Begins([Company Name],"g") OR Begins([Company Name],"h") OR Begins([Company Name],"i") OR Begins([Company Name],"j") OR Begins([Company Name],"k"), "E-K Clients",
Begins([Company Name],"l") OR Begins([Company Name],"m") OR Begins([Company Name],"n") OR Begins([Company Name],"o"), "L-O Clients",
Begins([Company Name],"p") OR Begins([Company Name],"q") OR Begins([Company Name],"r"), "P-R Clients",
Begins([Company Name],"s") OR Begins([Company Name],"t"), "S-T Clients",
Begins([Company Name],"u")
OR Begins([Company Name],"v")
OR Begins([Company Name],"w")
OR Begins([Company Name],"x")
OR Begins([Company Name],"y")
OR Begins([Company Name],"z"), "U-Z Clients")
