How to count the columns of a td showing the tatal in the end

Your code is a bit complicated to understand, but I think it will help.

Create a method to return the number of attendances and absences of each student, go through the alunos object using the forEach method, every loop make a filter on the presencas object using the filter method, it return a new array with the objects that passed the test, then retrieve the number of elements in that array.

calcularPresencas () {
  this.alunos.forEach((aluno, index) => {
    // Filtra por "cd_aluno" e pela "situacao" para presença
    this.alunos[index].totalPresente = this.presencas.filter(p => p.cd_aluno === aluno.cd_aluno && p.situacao === 'p').length;
    // Filtra por "cd_aluno" e pela "situacao" para falta
    this.alunos[index].totalFalta = this.presencas.filter(p => p.cd_aluno === aluno.cd_aluno && p.situacao === 'f').length;
  });
}

Run it on method ngOnInit() after this.getAlunos();

...
this.calcularPresencas();

Method of salvar(aluno, dia, situacao) filter on object presencas by cd_aluno and nu_dia , if the number of elements returned is greater than 0 it updates the existing situation, if otherwise it will add the object form to the object presencas . Then call the calcularPresencas method to update the data:

// Filtra por "cd_aluno" e "nu_dia"
let al = this.presencas.filter(p => p.cd_aluno == aluno.cd_aluno && p.nu_dia === dia);
//  Verifica se o número de elementos retornado é maior que "0"
(al.length > 0) ? al[0].situacao = situacao : this.presencas.push(form);
// Executa o método para atualizar os dados
this.calcularPresencas();

The template:

<!-- PRECISO SOMAR AS FALTAS -->
<td style="text-align: center">{{aluno.totalFalta}}</td>
<!-- PRECISO SOMAR AS PRESENÇAS -->
<td style="text-align: center">{{aluno.totalPresente}}</td>

Notice that I changed the order because it was wrong.

  

You can see it working at stackblitz .