Splitted sorted BAM files with samtools

Hello. I’m trying to use the command “sort” of samtools (ex: samtools sort alignment.bam -o alignment.sorted.bam), but for some reason, instead of generating one single sorted file (ex: alignment.sorted.bam) it generates many files (ex: alignment.sorted.bam.0000.bam, alignment.sorted.bam.0001.bam, alignment.sorted.bam.0002.bam, …, alignment.sorted.bam.0020.bam). The odd thing is that I had used the exact same command before for a few data of the same batch and it generated a single file. Does anyone have any idea what could be happening and how do I solve this? I’m also thinking of merging the files, if I can’t solve it. How do I find out the correct order to merge? Thanks in advance.