I didn't find any mention of character limit in the docs. Seems like a very common requirement.
How Can I add a 300 characters limit to this Textarea ?
https://stackblitz.com/edit/angular-izbcey?file=app/input-overview-example.ts
You can use like this for text
<mat-form-field hintLabel="Max 300 characters">
<input matInput #input maxlength="300" placeholder="Enter some input">
<mat-hint align="end">{{input.value?.length || 0}}/300</mat-hint>
</mat-form-field>
for textarea
<mat-form-field hintLabel="Max 10 characters">
<textarea #txtarea matInput [maxLength]="10" [placeholder]="label"></textarea>
</mat-form-field>