thirty
卅 represents the number thirty, formed by combining three tens (十) into a single character.
卅 is a ligature of three 十 (ten) characters, directly representing thirty. The form is a straightforward combination of its numeric parts.
See three 十 shapes merged together: ten, ten, ten makes thirty.
For ソウ, imagine a person counting thirty items and saying 'so many!' — 'so' cues ソウ, and the three tens reinforce the number.