Length of a UTF-8 byte sequence
Z80Golf, 19 14 bytes

00000000: 2f6f 3e10 37ed 6a3d 30fb ee07 c03c       /o&gt;.7.j=0....&lt;


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-5 bytes thanks to @Bubbler

Example with input 0x41-Try it online!
Assembly

Example with input 0xC2-Try it online!

Example with input 0xE0-Try it online!

Example with input 0xF4-Try it online!

Assembly:

;input: register a
;output: register a
byte_count:&#9;&#9;&#9;;calculate 7^(log2(255^a))||1
&#9;cpl&#9;&#9;&#9;;xor 255
&#9;ld l,a
&#9;log2:
&#9;&#9;ld&#9;a,16
&#9;&#9;scf
&#9;log2loop:
&#9;&#9;adc&#9;hl,hl
&#9;&#9;dec&#9;a
&#9;&#9;jr&#9;nc,log2loop
&#9;xor 7
&#9;ret nz
&#9;inc a


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